*nix stdin and stdout support

[jobbautista9]

Currently pmdplay doesn't seem to support accepting stdin as its MML input like this:

$ cat example.m2 | pmdplay -

Or saving the wav into stdout like this:

$ pmdplay example.m2 - | sox -t wav - -t mp3 example.mp3

I'd appreciate support for pipes into and out of pmdplay. This is the only good PMD player I could find that uses the command line.