Provide the user vote in manager

[philippeluickx]

Access the whole object.

[philippeluickx]

Related to #48

[shellfly]

What about model.votes.get(user_id) for this feature? I think get is more intuitive.
You also need to add some tests for this method.

[philippeluickx]

Will also add the method to the docs.

[philippeluickx]

Changes made, please review!

[codecov-io]

Codecov Report

Merging #52 into master will increase coverage by 0.07%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #52      +/-   ##
==========================================
+ Coverage   92.34%   92.41%   +0.07%     
==========================================
  Files           7        7              
  Lines         209      211       +2     
  Branches       14       14              
==========================================
+ Hits          193      195       +2     
  Misses         13       13              
  Partials        3        3

Impacted Files	Coverage Δ
vote/managers.py	92.3% <100%> (+0.13%)	⬆️
vote/__init__.py
vote/templatetags/__init__.py	100% <0%> (ø)

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