Character classes

(These are really just 2 more commits atop #325.)

Add proper, full parsing of bracketed character classes.

This will let us spit out Python and JS regexes so we can run Path- and RegexpFilters.

* Upgrade to parsimonious 0.6 for the `unwrapped_exceptions`.
* Add a min length arg to simplified() so we can use it to make tests more readable and writeable.