Add support for compression level in ZstdDictTrainer

[tomerr90]

@luben Sorry, not sure if I should have added you as a reviewer somehow

[luben]

LGTM, running the tests

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.90%. Comparing base (c76455c) to head (75f1fc2).
Report is 36 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #347      +/-   ##
============================================
- Coverage     60.01%   59.90%   -0.11%     
- Complexity      308      313       +5     
============================================
  Files            26       27       +1     
  Lines          1473     1524      +51     
  Branches        170      177       +7     
============================================
+ Hits            884      913      +29     
- Misses          434      451      +17     
- Partials        155      160       +5     

[tomerr90]

Thanks @luben !
When are you planning on publishing a new version?

[luben]

Yes, I will publish it later this week