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.
> don't have proper octal escape sequences!

There are octal escape sequences: "\XXX" where "XXX" is a number between 0 and 377 (dec 0-255). Do you mean a corresponding "\o" like the "\x" for hex? That would probably mean adding a corresponding "\b" for binary, but that's not allowed because "\b" is already backspace :\