TF-IDF backend, load command + necessary infrastructure

[osma]

No description provided.

[codecov]

Codecov Report

Merging #63 into master will decrease coverage by 0.54%.
The diff coverage is 93.3%.

@@            Coverage Diff             @@
##           master      #63      +/-   ##
==========================================
- Coverage   94.61%   94.07%   -0.55%     
==========================================
  Files          27       27              
  Lines         669      878     +209     
==========================================
+ Hits          633      826     +193     
- Misses         36       52      +16

Impacted Files	Coverage Δ
tests/test_project.py	100% <ø> (ø)	⬆️
tests/test_backend_http.py	100% <ø> (ø)	⬆️
annif/analyzer/snowball.py	100% <100%> (ø)
annif/corpus/subject.py	100% <100%> (ø)	⬆️
tests/test_backend.py	100% <100%> (ø)	⬆️
annif/backend/__init__.py	93.93% <100%> (+0.39%)	⬆️
tests/test_backend_tfidf.py	100% <100%> (ø)
annif/cli.py	90.21% <100%> (+2.17%)	⬆️
tests/test_analyzer.py	100% <100%> (ø)	⬆️
annif/project.py	100% <100%> (ø)	⬆️
... and 10 more

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[coveralls]

Coverage decreased (-0.5%) to 94.077% when pulling 2e6921e on tfidf-backend into dd7af41 on master.