Command line player a2play

Added a rudimentary command line player. It will try to play "Song"
by default, but the -p switch allows you to play any program in
the loaded module, with arguments.

There are also some basic object introspection capabilities; the -x
and -xr switches recursively list all objects in the last loaded
module or engine root respectively, printing out type, period, size
etc of waves, and I/O port counts and control registers of units.

Closes #20, though there is no file writing support yet.