Ignore n levels instead of just leaves

[incaseoftrouble]

WIP

I still have to actually test this but small bump to get some feedback :)

Fixes #51

(After this I can also implement #50 )

[blingenf]

Thank you! A few comments:

• I think ignore_leaf will have to become ignore_depth on line 120 of __main__.py
• for API changes (changing type to file_type and ignore_leaf to ignore_depth) I try to keep the old parameters working for one version with a DeprecationWarning. But I can add that myself after this is merged -- I think the parameter changes are sensible.
• The new parameter should be documented in README.md and docs/cmdline.rst
• Each parameter for the detector should have a relevant unit test in tests/test_detector.py

But in general where / how this is implemented looks good to me. Once it's tested and the unit tests are passing I should be good to merge.

[incaseoftrouble]

Sure, didn't get to it this week :-)

I also added a get_comparison_pairs() to separate out the "ignore pair" logic -- this made it sensible to let the progress bar go over these pairs directly instead of test files (but this I can revert).