Enhancements to vw_multi backend

[osma]

• Allow use of log_multi and multilabel_oaa algorithms
• allow setting ngram and probabilities options
  Vowpal Wabbit regular backend #230

[codecov]

Codecov Report

Merging #254 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #254      +/-   ##
==========================================
+ Coverage    99.3%   99.31%   +<.01%     
==========================================
  Files          51       51              
  Lines        2432     2466      +34     
==========================================
+ Hits         2415     2449      +34     
  Misses         17       17

Impacted Files	Coverage Δ
tests/test_backend_vw_multi.py	100% <100%> (ø)
annif/backend/vw_multi.py	100% <100%> (ø)	⬆️

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