Provide regexp units through Python bindings

[metthal]

Regular expressions are parsed down to individual regular expression units (character class, alternation, meta characters like ., \s, ...). These unit are available through C++ API but they are not available through Python bindings. It would be nice to have them available in case anybody wants to process them from Python.

It should be enough to add all of them into src/python/yaramod_python.cpp with pointers to their respective C++ counterparts.