Add instance variables for other sprites in the sprite key

[arnavb]

Fixes #17

Since adding support for new sprites shouldn't change how sprites are regularly dealt with, this change should be backwards compatible. Once merged, this will be released as v1.5.0.

TODO:

• [x] Fix issue so that other sprites don't conflict with normal sprites.
• [x] Add keys to Pokemon class for other sprites.
• ~Update tests with more up to date sample Pokemon and be more exhaustive~. Will be done in another PR.
• [x] Update documentation.

[codecov[bot]]

Codecov Report

Merging #18 into master will decrease coverage by 5.47%. The diff coverage is 70.37%.

@@             Coverage Diff             @@
##            master      #18      +/-   ##
===========================================
- Coverage   100.00%   94.52%   -5.48%     
===========================================
  Files            4        4              
  Lines          126      146      +20     
  Branches        20       26       +6     
===========================================
+ Hits           126      138      +12     
- Misses           0        6       +6     
- Partials         0        2       +2     

Impacted Files	Coverage Δ
pypokedex/api.py	100.00% <ø> (ø)
pypokedex/pokemon.py	92.59% <70.37%> (-7.41%)	:arrow_down:

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