Allow video_index, resolution and fps to be specified on commandline

[bobek]

This will allow for use on different computers and with weird USB cameras.

My USB camera is aliexpress breed, which is quite picky about resolution+fps combinations. Also as I am using notebook as a device for doing analysis, the video0 is typically internal camera.

I came across #13 only after introducing these changes, but approach seems to be quite compatible. I would also like to second @rafl question about code organization and suggest having just one file pyspectrometer.py and use git tags for version tracking.

[ckuethe]

Aside from your comments, I came up with basically the very same diff this morning, though I just used .lower().split('x') for the resolution parsing.

I'm using my laptop which has an internal webcam, and indeed my external camera is something cheap and terrible that doesn't like 25fps but if I skip setting capture rate then it behaves quite sensibly.

I recommend merging this diff, cuz I have a few more changes to go on top of it :)