Overhaul matching of percent delimited literals

This adds new types and improves the scoping to better match the contents, includes several test cases in a new test file.

One limitation I am aware of is that `%q foo ` is not recognized as a string when it should be. Allowing whitespace as a delimiter though adds a lot of ambiguity I don't think is worth getting into.

Based on patches from @pelargir, @noniq, and @elia, thanks to @elia for testing help. Fixes #41.

Author: infininight