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/operation.go.tmpl

@@ -11,7 +11,7 @@ package {{.Config.Package}}
 {{range .Operations}}
 {{.Doc}}
 func {{.Name}}(
-    {{if $.Config.ContextType -}}
+    {{if ne $.Config.ContextType "-" -}}
     ctx {{ref $.Config.ContextType}},
     {{end}}
     {{- if not $.Config.ClientGetter -}}
@@ -47,7 +47,7 @@ func {{.Name}}(
 
     var err error
     {{if $.Config.ClientGetter -}}
-    client, err := {{ref $.Config.ClientGetter}}({{if $.Config.ContextType}}ctx{{else}}{{end}})
+    client, err := {{ref $.Config.ClientGetter}}({{if ne $.Config.ContextType "-"}}ctx{{else}}{{end}})
     if err != nil {
         return nil, err
     }
@@ -55,7 +55,7 @@ func {{.Name}}(
 
 	var retval {{.ResponseName}}
     err = client.MakeRequest(
-        {{if $.Config.ContextType}}ctx{{else}}nil{{end}},
+        {{if ne $.Config.ContextType "-"}}ctx{{else}}nil{{end}},
         "{{.Name}}",
         `{{.Body}}`,
         &retval,