708 - Add Natural Key Support to Tag model.

[Trafire]

#708

Adds the ability to load/dump Tags by by natural keys rather thank PK

[codecov]

Codecov Report

Attention: Patch coverage is 90.90909% with 2 lines in your changes missing coverage. Please review.

Project coverage is 92.21%. Comparing base (2ecc9c7) to head (0de8a38).
Report is 4 commits behind head on master.

Files	Patch %	Lines
taggit/models.py	90.90%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #905      +/-   ##
==========================================
+ Coverage   91.44%   92.21%   +0.77%     
==========================================
  Files           9        9              
  Lines         736      758      +22     
  Branches      116      137      +21     
==========================================
+ Hits          673      699      +26     
+ Misses         41       37       -4     
  Partials       22       22              

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[tim-schilling]

@rtpg when you have time, can you apply the djangonauts-reserved label to the following PRs (github allows you to mass apply labels from the list view):

• https://github.com/jazzband/django-taggit/pulls?q=is%3Apr+author%3ATrafire
• https://github.com/jazzband/django-taggit/pulls?q=is%3Apr+author%3Aguel-codes