Add extraction mode to skew images

It's the reverse of Extraction.deskew_min_area_rect.

The idea is to create skewed text lines for training data so kaldi would be more robust to skewed lines, because sometimes there aren't any skewed lines in the training datasets. https://redmine.teklia.com/issues/737

So in this extraction mode, the min area rect should be extracted and then rotated by X degrees (given as a parameter).