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

go.mod

@@ -4,7 +4,9 @@ go 1.13
 
 require (
 	github.com/bradleyjkemp/cupaloy/v2 v2.6.0
+	// Should match golangci-lint version in .github/workflows/go.yml
+	github.com/golangci/golangci-lint v1.42.0
 	github.com/vektah/gqlparser/v2 v2.1.0
-	golang.org/x/tools v0.0.0-20190125232054-d66bd3c5d5a6
-	gopkg.in/yaml.v2 v2.2.4
+	golang.org/x/tools v0.1.5
+	gopkg.in/yaml.v2 v2.4.0
 )