Add --no-hash option to mkogg.py.

Add --no-hash option to mkogg.py.

Hashing is slow, so now you can avoid computing hashes to determine if
the target file has changed.  You can miss small changes this way
(e.g. tag changes in the source, or changed tag handling in mkogg.py
itself), but sometimes I don't care about that enough to want to hash
my whole music collection ;).

I've also added a few more ignored and translated ID3v2 keys.