Handle license

[ClemDoum]

Description:
In some case the dataset might contain data subject to license terms that need to be placed next to the parsers built from this data.

When fitting the custom entity parser, if the dataset contains license information, the gazetteer entity parser are build with license_info (snips-nlu-parser>=0.3.1) which ensure that the license will be persisted at the right location when persisting the engine

Checklist:

• My PR is ready for code review
• I have added some tests, if applicable, and run the whole test suite, including linting tests
• I have updated the documentation, if applicable

[codecov-io]

Codecov Report

Merging #832 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #832      +/-   ##
==========================================
+ Coverage   88.95%   88.96%   +0.01%     
==========================================
  Files          79       79              
  Lines        5090     5095       +5     
  Branches      966      968       +2     
==========================================
+ Hits         4528     4533       +5     
  Misses        417      417              
  Partials      145      145