Add a flag --init to write a default config (#81)

## Summary:
Steve pointed out (#73) that having genqlient with no arguments silently
use a default config file was a bit confusing, and changed it to use
`genqlient.yaml` by default (#74).  Mark pointed out (#76) that this
makes it a bit less convenient when you're starting from scratch; you
have to go create a config file.  In this commit I add a new init flag
that creates you a config file before using it.

Originally the suggestion was to use subcommands, e.g. we'd have
`genqlient init` and `genqlient generate` and so on.  But I couldn't
think of anything else we might want subcommands for in the future, and
it felt a little silly to make you type `generate` each time.  So
instead, I made it a flag, which has the nice property that you can do
`genqlient --init` and it will generate and then use a config file.  (I
mean, maybe it will immediately crash because you don't have a schema,
but hopefully that's still a useful clue as to what to do next!)  The
implmentation was fairly trivial.

Since we now have a nice way to generate a default config, I removed the
default values for most of the options; I've always felt they were
probably more confusing than helpful.  (And indeed, all the users I know
of (Khan/webapp, and the much smaller project Steve was working on, are
setting those options explicitly.)  This required a slight change to
the syntax to say "don't use context", which is probably also net clearer.

I decided this is also a good time to pull in a proper CLI parser (#31);
see ADR-504 for more on that choice.  This also adds some nice help
messages!

Fixes #76, #31.

Issue: https://github.com/Khan/genqlient/issues/76

## Test plan:
```
go run .
go run . --init
go run . --init example/genqlient.yaml     # refuses to clobber
go run . --init example/newgenqlient.yaml
```


Author: benjaminjkraft

Reviewers: dnerdy, aberkan, MiguelCastillo, StevenACoffman

Required Reviewers: 

Approved By: dnerdy

Checks: ✅ Test (1.17), ✅ Test (1.16), ✅ Test (1.15), ✅ Test (1.14), ✅ Lint, ✅ Test (1.17), ✅ Test (1.16), ✅ Test (1.15), ✅ Test (1.14), ✅ Lint

Pull Request URL: https://github.com/Khan/genqlient/pull/81

generate/config.go

@@ -2,19 +2,14 @@ package generate
 
 import (
 	"go/token"
+	"io"
 	"io/ioutil"
+	"os"
 	"path/filepath"
 
 	"gopkg.in/yaml.v2"
 )
 
-var defaultConfig = &Config{
-	Schema:      "schema.graphql",
-	Operations:  []string{"genqlient.graphql"},
-	Generated:   "generated.go",
-	ContextType: "context.Context",
-}
-
 type Config struct {
 	// The filename with the GraphQL schema (in SDL format); defaults to
 	// schema.graphql
@@ -56,9 +51,9 @@ type Config struct {
 	// Set to the fully-qualified name of a Go type which generated helpers
 	// should accept and use as the context.Context for HTTP requests.
 	//
-	// Defaults to context.Context; set to the empty string to omit context
-	// entirely (i.e. use context.Background()).  Must be a type which
-	// implements context.Context.
+	// Defaults to context.Context; set to "-" to omit context entirely (i.e.
+	// use context.Background()).  Must be a type which implements
+	// context.Context.
 	ContextType string `yaml:"context_type"`
 
 	// If set, a function to get a graphql.Client, perhaps from the context.
@@ -157,6 +152,10 @@ func (c *Config) ValidateAndFillDefaults(configFilename string) error {
 		c.ExportOperations = filepath.Join(c.baseDir(), c.ExportOperations)
 	}
 
+	if c.ContextType == "" {
+		c.ContextType = "context.Context"
+	}
+
 	if c.Package == "" {
 		abs, err := filepath.Abs(c.Generated)
 		if err != nil {
@@ -175,23 +174,36 @@ func (c *Config) ValidateAndFillDefaults(configFilename string) error {
 }
 
 func ReadAndValidateConfig(filename string) (*Config, error) {
-	config := *defaultConfig
-	if filename != "" {
-		text, err := ioutil.ReadFile(filename)
-		if err != nil {
-			return nil, errorf(nil, "unreadable config file %v: %v", filename, err)
-		}
-
-		err = yaml.UnmarshalStrict(text, &config)
-		if err != nil {
-			return nil, errorf(nil, "invalid config file %v: %v", filename, err)
-		}
+	text, err := ioutil.ReadFile(filename)
+	if err != nil {
+		return nil, errorf(nil, "unreadable config file %v: %v", filename, err)
 	}
 
-	err := config.ValidateAndFillDefaults(filename)
+	var config Config
+	err = yaml.UnmarshalStrict(text, &config)
+	if err != nil {
+		return nil, errorf(nil, "invalid config file %v: %v", filename, err)
+	}
+
+	err = config.ValidateAndFillDefaults(filename)
 	if err != nil {
 		return nil, errorf(nil, "invalid config file %v: %v", filename, err)
 	}
 
 	return &config, nil
 }
+
+func initConfig(filename string) error {
+	// TODO(benkraft): Embed this config file into the binary, see
+	// https://github.com/Khan/genqlient/issues/9.
+	r, err := os.Open(filepath.Join(thisDir, "default_genqlient.yaml"))
+	if err != nil {
+		return err
+	}
+	w, err := os.OpenFile(filename, os.O_WRONLY|os.O_CREATE|os.O_EXCL, 0o644)
+	if err != nil {
+		return err
+	}
+	_, err = io.Copy(w, r)
+	return err
+}