Parsing: Allow both harmony and custom esprima versions.

Thought about that.

Passing harmony in here means users of `StringChecker` directly can't get es6 support without bringing their own version of esprima.

I agree the maintenance costs are slightly higher to have a boolean option, but this makes it easiest for our users.

If we're concerned that our built in harmony will get out of date, we can always tell users to bring their own version, but I don't want to lose this convenience.

Thoughts?