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

generate/operation.go.tmpl

@@ -43,9 +43,18 @@ func {{.Name}}(
     }
     {{end}}
     {{end}}
+    {{end -}}
+
+    var err error
+    {{if $.Config.ClientGetter -}}
+    client, err := {{ref $.Config.ClientGetter}}({{if $.Config.ContextType}}ctx{{else}}{{end}})
+    if err != nil {
+        return nil, err
+    }
     {{end}}
+
 	var retval {{.ResponseName}}
-    err := {{if $.Config.ClientGetter}}{{$.Config.ClientGetter}}{{else}}client{{end}}.MakeRequest(
+    err = client.MakeRequest(
         {{if $.Config.ContextType}}ctx{{else}}nil{{end}},
         "{{.Name}}",
         `{{.Body}}`,