First implementation of VW regular backend

[osma]

Part of #230

Implements a vw_multi backend that currently only supports a VW one-against-all (oaa) classifier with a limited set of parameters. No support for online learning yet.

[codecov]

Codecov Report

Merging #249 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #249      +/-   ##
==========================================
+ Coverage   99.24%   99.28%   +0.04%     
==========================================
  Files          48       51       +3     
  Lines        2252     2386     +134     
==========================================
+ Hits         2235     2369     +134     
  Misses         17       17

Impacted Files	Coverage Δ
annif/backend/fasttext.py	98.59% <100%> (-0.22%)	⬇️
annif/backend/__init__.py	100% <100%> (ø)	⬆️
tests/test_backend_vw.py	100% <100%> (ø)
annif/backend/mixins.py	100% <100%> (ø)
annif/backend/vw_multi.py	100% <100%> (ø)

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[osma]

This pull request introduces 1 alert when merging 92e737e into d8a4d26 - view on LGTM.com

new alerts:

• 1 for Unused import

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Comment posted by LGTM.com