Support escaped backticks in template literals

This is a totally valid use but since the lexer just looks for the next
backtick as a marker for the end of the literal, things fall apart
quickly.

I added this use to the fixture, as well as one for literals without templates... just in case.
I didn't have the `isTemplateLiteral` check originally but it broke existing behavior of jshint, which warns if you escape the backtick in a string literal. I think that's a fine warning (it's not actually a "bad" escape, but it is unnecessary). However it would be incorrect to warn for that while processing a template literal since that is the only way you can put a literal backtick in there. So I think we need the check.

And actually, this should also look for $ (and lookahead for {), since you would need to escape that too. But that needs more work (we shouldn't put ourselves into the inTemplate state) than I want to put into this.