enable "extract --recursive"

[boyanpenkov]

papers extract thing.pdf is wonderful for checking that the DOI/crossref call on a single PDF is going to make sense.

This PR enables the same behavior on a whole directory recursively, much like papers add --recursive.

@perrette I need a pointer on how to write good tests for this one -- I was playing with test/test_extract.py but can't figure out how to capture printed output to compare to the database of the two test files you have -- much like in tests/test_add.py

[boyanpenkov]

aca948e the last one here is a pointer to how I think this should go, but I wanted to see about the print capture....

[boyanpenkov]

Note that the race condition here is visible also if you run multiple concurrent copies of papers under separate interpreters.

[boyanpenkov]

OK, this does have a test that does work on the existing test files and is the bare minimum of quality control, but I'd be glad for a pointer to a better way of doing that print capture.

Ready for review.