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

generate/parse.go

@@ -4,7 +4,6 @@ import (
 	"fmt"
 	goAst "go/ast"
 	goParser "go/parser"
-	"go/token"
 	goToken "go/token"
 	"io/ioutil"
 	"path/filepath"
@@ -134,7 +133,7 @@ func getQueriesFromGo(text string, basedir, filename string) ([]*ast.QueryDocume
 		}
 
 		basicLit, ok := node.(*goAst.BasicLit)
-		if !ok || basicLit.Kind != token.STRING {
+		if !ok || basicLit.Kind != goToken.STRING {
 			return true // recurse
 		}