Automatic crop

[rmmh]

ffmpeg has a 'cropdetect' video filter that will suggest cropping bounds based on where black bars in the video are. This could be use directly, or re-implemented -- it's a very simple thresholding algorithm.

Cf. #24

[WebMBro]

Pretty good idea, I guess I'll add either a button or a checkbox to the cropping form for autocropping.