Fix continuations in preprocessor definitions

This fixes the issues with line continuations I reported in atom/atom#2528. At least it seems like it does, I don't really have all that much experience with regular expressions as advanced as the ones used in this grammar so I'm not sure if this introduces any other bugs.
Did you try using `(?<!\\\\)(?=$)`? Just curious if there was any difference using `\n` vs. `$`