Add output argument

[averwhy]

Quickly modified inference.py to support a specification of an output argument. If an argument is specified, it checks if the output folder exists and creates one if not. e.g:

inference.py --input song.mp3 --output_dir results

would create a folder named results (if it was not already existing) and put the resulting separated audio files into there.

Tested & working:

• [x] Just audio with output dir specified & non-existing
• [x] Audio and image with output dir specified & non-existing
• [x] Just audio with output dir specified & existing
• [x] Audio and image with output dir specified & existing

[tsurumeso]

Thanks for your contribution! Please check the comments.

[averwhy]

Sorry, which comments?

[tsurumeso]

This is it.

[averwhy]

Oh strange, i cant see that on my screen. I will make those changes though.

[averwhy]

These changes are working.

[tsurumeso]

LGTM!