Pass-throughs should not use `-map`

This currently produces the wrong ffmpeg args:

>>> ' '.join(ffmpeg
...     .merge_outputs(
...         (ffmpeg
...             .input('in1.mp4')
...             .output('out1.mp4')
...         ),
...         (ffmpeg
...             .input('in2.mp4')
...             .output('out2.mp4')
...         )
...     )
...     .get_args()
... )
u'-i in1.mp4 -i in2.mp4 out1.mp4 -map [1] out2.mp4'

The -map should not be there, since no -filter_complex param is needed.

Should be fairly easy to fix by omitting -map for passthroughs and generating output params in the correct order.

See test_multi_passthrough in feature/17 (or master once #17 (closed) is merged).