Experiment with adaptive thresholding

This produces better results on a wider range of scans but wider
testing is needed
extract-figures: add more interactive progress information

This is good for avoiding the need to write down the final settings
and once the config refactor happens we can generate a nice command-
line to copy and paste.
Figure extraction: merge overlapping rectangles

This is a simple start – it likely makes sense to merge rectangles
which do not overlap but which are aligned closely along one edge