Add docstring for _Buffer.extension.
Update rosp.Package constructor to accept name or path.
Migrate BufInit and BufFiletype functions from VimL to Python.
Turn rosvim.py into a module. Add FunctionManager class (successor of function decorator).
Improve vimp._Options so that it can handle decorated functions.

Now an option could be assigned with a function. This function has to
be decorated by FunctionManager beforehand.
Move filetype detection, completion, and stuff into a separate module.