bsatool should be able to create BSA archives, not only to extract it

Morrowind game resources are packed into several BSA archives. OpenMW can read BSA, but currently bsatool doesn't support creating new archives. BSA can be created with open-source tool BSArch, but to make OpenMW/OpenMW-CS self-sufficient it would be good to add such functionality to our bsatool.

I've added a label "beginner" because this task doesn't require deep knowledge about OpenMW internals, but it doesn't mean that it is simple.