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/main.go

@@ -6,6 +6,8 @@ import (
 	"os"
 	"path/filepath"
 	"strings"
+
+	"github.com/alexflint/go-arg"
 )
 
 func readConfigGenerateAndWrite(configFilename string) error {
@@ -36,25 +38,32 @@ func readConfigGenerateAndWrite(configFilename string) error {
 	return nil
 }
 
+type cliArgs struct {
+	ConfigFilename string `arg:"positional" placeholder:"CONFIG" default:"genqlient.yaml" help:"path to genqlient configuration (default genqlient.yaml)"`
+	Init           bool   `arg:"--init" help:"write out and use a default config file"`
+}
+
+func (cliArgs) Description() string {
+	return strings.TrimSpace(`
+Generates GraphQL client code for a given schema and queries.
+See https://github.com/Khan/genqlient for full documentation.
+`)
+}
+
 func Main() {
-	var err error
-	defer func() {
+	exitIfError := func(err error) {
 		if err != nil {
 			fmt.Println(err)
 			os.Exit(1)
 		}
-	}()
-
-	switch len(os.Args) {
-	case 2:
-		err = readConfigGenerateAndWrite(os.Args[1])
-	case 1:
-		err = readConfigGenerateAndWrite("genqlient.yaml")
-	default:
-		argv0 := os.Args[0]
-		if strings.Contains(argv0, string(filepath.Separator)+"go-build") {
-			argv0 = "go run github.com/Khan/genqlient"
-		}
-		err = errorf(nil, "usage: %s [config]", argv0)
 	}
+
+	var args cliArgs
+	arg.MustParse(&args)
+	if args.Init {
+		err := initConfig(args.ConfigFilename)
+		exitIfError(err)
+	}
+	err := readConfigGenerateAndWrite(args.ConfigFilename)
+	exitIfError(err)
 }