Ignore subdirectories in input directory

[melodell]

(To support HTML dataset changes to P5)

We want Madoop to ignore subdirectories in the input directory (and not throw an error).

Note to @awdeorio: We're gonna need a new release before P5 launches!

[codecov[bot]]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Files	Coverage Δ
madoop/mapreduce.py	96.17% <50.00%> (-0.60%)	:arrow_down:

:loudspeaker: Thoughts on this report? Let us know!.

[awdeorio]

Subsumed by #67