Enable golangci-lint (#49)

## Summary:
It would be nice to have some linting beyond `go vet`!  Now we do.  I
started by copying the config from Khan/webapp.  I did remove a couple
of staticcheck checks that I didn't feel were useful.  (Note also that
exportloopref is the replacement for scopelint in newer golangci-lint.)

Included are all the needed lint fixes; most are stylistic but the
changes in the example are a (minor) bugfix.

Fixes #22.
Issue: https://github.com/Khan/genqlient/issues/22

## Test plan:
make check

Author: benjaminjkraft

Reviewers: aberkan, dnerdy, benjaminjkraft, csilvers, MiguelCastillo

Required Reviewers: 

Approved by: aberkan, 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/49

example/caller.go

@@ -44,7 +44,8 @@ func Main() {
 
 	switch len(os.Args) {
 	case 1:
-		viewerResp, err := getViewer(context.Background(), graphqlClient)
+		var viewerResp *getViewerResponse
+		viewerResp, err = getViewer(context.Background(), graphqlClient)
 		if err != nil {
 			return
 		}
@@ -52,7 +53,8 @@ func Main() {
 
 	case 2:
 		username := os.Args[1]
-		userResp, err := getUser(context.Background(), graphqlClient, username)
+		var userResp *getUserResponse
+		userResp, err = getUser(context.Background(), graphqlClient, username)
 		if err != nil {
 			return
 		}