Clean up example a little (#89)

## Summary:
I realized:
- it's simpler to avoid the usual `cmd` and just put it all in the same
  directory
- we don't need schema.json since GitHub now posts a schema in SDL
  format

## Test plan:
make lint example


Author: benjaminjkraft

Reviewers: dnerdy, StevenACoffman, aberkan, jvoll, mahtabsabet, MiguelCastillo

Required Reviewers: 

Approved By: dnerdy, StevenACoffman

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/89

example/main.go

@@ -0,0 +1,68 @@
+package main
+
+import (
+	"context"
+	"fmt"
+	"net/http"
+	"os"
+
+	"github.com/Khan/genqlient/graphql"
+)
+
+type authedTransport struct {
+	key     string
+	wrapped http.RoundTripper
+}
+
+func (t *authedTransport) RoundTrip(req *http.Request) (*http.Response, error) {
+	req.Header.Set("Authorization", "bearer "+t.key)
+	return t.wrapped.RoundTrip(req)
+}
+
+func main() {
+	var err error
+	defer func() {
+		if err != nil {
+			fmt.Println(err)
+			os.Exit(1)
+		}
+	}()
+
+	key := os.Getenv("GITHUB_TOKEN")
+	if key == "" {
+		err = fmt.Errorf("must set GITHUB_TOKEN=<github token>")
+		return
+	}
+
+	httpClient := http.Client{
+		Transport: &authedTransport{
+			key:     key,
+			wrapped: http.DefaultTransport,
+		},
+	}
+	graphqlClient := graphql.NewClient("https://api.github.com/graphql", &httpClient)
+
+	switch len(os.Args) {
+	case 1:
+		var viewerResp *getViewerResponse
+		viewerResp, err = getViewer(context.Background(), graphqlClient)
+		if err != nil {
+			return
+		}
+		fmt.Println("you are", viewerResp.Viewer.MyName, "created on", viewerResp.Viewer.CreatedAt.Format("2006-01-02"))
+
+	case 2:
+		username := os.Args[1]
+		var userResp *getUserResponse
+		userResp, err = getUser(context.Background(), graphqlClient, username)
+		if err != nil {
+			return
+		}
+		fmt.Println(username, "is", userResp.User.TheirName, "created on", userResp.User.CreatedAt.Format("2006-01-02"))
+
+	default:
+		err = fmt.Errorf("usage: %v [username]", os.Args[0])
+	}
+}
+
+//go:generate go run github.com/Khan/genqlient genqlient.yaml