Correctly recognize ? character quote tokens

This introduces a syntax-propertize function that will ferret out
non-constituent ? characters and apply the generic string delimiter
syntax text property to it and the character it escapes. (This handles
?\# sequences too.)

Fixes #26.
Add (failing) test for last comment in a block (#27)
Fontify character literals as strings.

Since we're using a syntax property function, we can attempt to be
smarter about parser state. I'd like to extend this to sigil
expressions, also (which I suspect might be a possible way to fix
#23).
Merge pull request #28 from antifuchs/recognize-qm-char-syntax

Recognize ? char syntax