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.
@zpao I'd like this to be a complete fix and I think that part of that fix is to address the existing incorrect parts: 

> broke existing behavior of jshint, which warns if you escape the backtick in a string literal.