Adding support for multiple custom files

Works in a few ways:
 * a single file: --custom foo.py (as before)
 * comma-separated list: --custom foo.py,bar.py
 * multiple custom args: --custom foo.py --custom bar.py
You could probably make `custom` a method and then use

    callback=partial( self.custom, self)

or alternately maybe something like

    callback=MininetRunner.custom.__get__( self, MininetRunner )