tools/pyboard.py: Add filesystem commands to ls/cat/cp/rm remote files.

Use "-f" to select filesystem mode, followed by the command to execute.
Optionally put ":" at the start of a filename to indicate that it's on the
remote device, if it would otherwise be ambiguous.

Examples:

    $ pyboard.py -f ls
    $ pyboard.py -f cat main.py
    $ pyboard.py -f cp :main.py .   # get from device
    $ pyboard.py -f cp main.py :    # put to device
    $ pyboard.py -f rm main.py