Initial support for regular expressions

[zmaril]

By adding in a parser for regular expression syntax as well as transforms that convert the parsed regular expression output into instaparse combinators, we can use instagenerate on grammars that contain regular expressions. This commit adds in some simple support for the basics of regular experssions, as well as adding some tests.

[zmaril]

Work in progress, but need to implement negative lookahead inside instagenerate.