Add fastText backend. Fixes #74

[osma]

No description provided.

[coveralls]

Coverage increased (+0.09%) to 95.799% when pulling f6bff45 on fasttext-backend into b8def22 on master.

[codecov]

Codecov Report

Merging #92 into master will increase coverage by 0.09%.
The diff coverage is 96.72%.

@@            Coverage Diff             @@
##           master      #92      +/-   ##
==========================================
+ Coverage    95.7%   95.79%   +0.09%     
==========================================
  Files          30       32       +2     
  Lines        1211     1333     +122     
==========================================
+ Hits         1159     1277     +118     
- Misses         52       56       +4

Impacted Files	Coverage Δ
annif/backend/backend.py	90.62% <100%> (+0.62%)	⬆️
annif/backend/tfidf.py	100% <100%> (ø)	⬆️
annif/backend/__init__.py	88.37% <100%> (+0.56%)	⬆️
tests/test_backend_fasttext.py	100% <100%> (ø)
annif/backend/fasttext.py	100% <100%> (ø)
annif/project.py	90% <33.33%> (-2.99%)	⬇️

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