Added callback parameter to extract and extractall

[nekosaur]

Don't know if this is worth anything, but to make things easy for myself I added a callback parameter to rarfile.extract and extractall, since handles are not exposed outside of unrarlib.

[matiasb]

Right, this isn't exposed in RarFile because it is kind of low-level and somehow tied to the ctypes wrapper implementation. Will give it some thought, though. I agree it would be interesting to have the feature available (maybe making it accesible through RarFile in a more pythonic way). Thanks!