prodos-utils: Support adding .BAS files #16

I tried to use prodos to SAVE a BAS file onto a ProDOS disk, but it didn't work.

One part of the problem is that BAS files need an annotation/subtype of 0x0801, but prodos only respects -a for files of type BIN. However, even when I added a new condition at around line 464, in prodos_add_file(), the contents of the BASIC file installed appeared to be a jumble of garbage when I attempted to load it in an emulator. Wasn't clued into how things work enough to pursue it any further.