Clean up, test, and document ContextType and ClientGetter options (#77)

## Summary:
ContextType is in use at Khan as a part of our ka-context system; it
basically just lets you configure the type to pass as the `ctx` argument
to genqlient helpers (or say to omit such an argument).  ClientGetter I
wrote thinking we might use it; then we didn't (because we have a few
different clients we may use) but it's not much code and may be helpful
to others.  In this commit I clean up, document, and add tests for both
options.

The cleanup is mainly for ClientGetter, which was kind of broken before
because it was a Go snippet but couldn't specify imports.  I was
thinking maybe you want to be able to write `ctx.Something()`, but I
just don't see how to make it work, so I made it a function of context,
which is probably the better idea anyway.

Additionally, I improved the documentation for both, and added tests for
those and several other config options that weren't completely tested.

Fixes #5.

Issue: https://github.com/Khan/genqlient/issues/5

## Test plan:
make check


Author: benjaminjkraft

Reviewers: dnerdy, aberkan, MiguelCastillo

Required Reviewers: 

Approved By: dnerdy

Checks: ✅ Test (1.17), ✅ Test (1.16), ✅ Test (1.15), ✅ Test (1.14), ✅ Test (1.13), ✅ Lint, ✅ Test (1.17), ✅ Test (1.16), ✅ Test (1.15), ✅ Test (1.14), ✅ Test (1.13), ✅ Lint

Pull Request URL: https://github.com/Khan/genqlient/pull/77

internal/testutil/types.go

@@ -1,5 +1,11 @@
 package testutil
 
+import (
+	"context"
+
+	"github.com/Khan/genqlient/graphql"
+)
+
 type ID string
 
 type Pokemon struct {
@@ -10,3 +16,13 @@ type Pokemon struct {
 func (p Pokemon) Battle(q Pokemon) bool {
 	return p.Level > q.Level
 }
+
+type MyContext interface {
+	context.Context
+
+	MyMethod()
+}
+
+func GetClientFromNowhere() (graphql.Client, error)                    { return nil, nil }
+func GetClientFromContext(ctx context.Context) (graphql.Client, error) { return nil, nil }
+func GetClientFromMyContext(ctx MyContext) (graphql.Client, error)     { return nil, nil }