Raise ParseError instead of displaying '<<< >>>'
Properly reindent recent additions.
Preliminary support for group references.

This allows both capturing groups (only provided if you pass in a dict),
and backreferences (which get an anonymous dict for the duration of the
lookup).  In my benchmarking, this is about 1.5% slower than the
previous version, while being much more powerful.
Update docs about capture group extraction.
Add AllStrings/AllMatches, update docs.
Rename Values to AllStrings in tests, as it is the new API
Merge branch 'backrefs'

This adds support for reading capture groups of the generated matches, and the
ability to refer to backrefs within the regex itself.