Added Doctest Testing

[christopherbunn]

Resolves #218

[codecov]

Codecov Report

Merging #233 into master will increase coverage by 0.33%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #233      +/-   ##
=========================================
+ Coverage   96.57%   96.9%   +0.33%     
=========================================
  Files          92      92              
  Lines        2422    2422              
=========================================
+ Hits         2339    2347       +8     
+ Misses         83      75       -8

Impacted Files	Coverage Δ
evalml/guardrails/utils.py	96% <ø> (ø)	⬆️
evalml/models/apply.py	25% <0%> (+25%)	⬆️

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