Subject corpus functionality

[osma]

No description provided.

[codecov]

Codecov Report

Merging #58 into master will increase coverage by 0.58%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #58      +/-   ##
==========================================
+ Coverage   94.02%   94.61%   +0.58%     
==========================================
  Files          26       27       +1     
  Lines         603      669      +66     
==========================================
+ Hits          567      633      +66     
  Misses         36       36

Impacted Files	Coverage Δ
tests/test_corpus.py	100% <100%> (ø)	⬆️
annif/corpus/__init__.py	100% <100%> (ø)	⬆️
annif/corpus/subject.py	100% <100%> (ø)
annif/corpus/docdir.py	100% <100%> (ø)	⬆️
annif/cli.py	88.04% <100%> (ø)	⬆️

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[coveralls]

Coverage increased (+0.6%) to 94.619% when pulling d441897 on subject-corpus into 2334d57 on master.