Add support for interfaces, part 3: automatically add __typename (#56)

## Summary:
In #52, I added support for interface types, but with the simplifying
restriction (among others) that the user must request the field
`__typename`.  In this commit, I remove this restriction.

The basic idea is simple: we preprocess the query to add `__typename`.
The implementation isn't much more complicated!  Although it required
some new wiring in a few places.

Issue: https://github.com/Khan/genqlient/issues/8

## 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/56

generate/genqlient_directive.go

@@ -146,6 +146,10 @@ func (g *generator) parsePrecedingComment(
 	pos *ast.Position,
 ) (comment string, directive *GenqlientDirective, err error) {
 	directive = new(GenqlientDirective)
+	if pos == nil || pos.Src == nil { // node was added by genqlient itself
+		return "", directive, nil // treated as if there were no comment
+	}
+
 	var commentLines []string
 	sourceLines := strings.Split(pos.Src.Input, "\n")
 	for i := pos.Line - 1; i > 0; i-- {