Add doctests to setup.cfg

[gsheni]

• Make it so that pytest runs doctests in docstrings

[codecov]

Codecov Report

Merging #491 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #491      +/-   ##
==========================================
+ Coverage   96.25%   96.26%   +<.01%     
==========================================
  Files         104      104              
  Lines        8714     8727      +13     
==========================================
+ Hits         8388     8401      +13     
  Misses        326      326

Impacted Files	Coverage Δ
featuretools/entityset/timedelta.py	86.42% <ø> (ø)	⬆️
featuretools/entityset/entity.py	96.1% <ø> (ø)	⬆️
featuretools/entityset/entityset.py	95.04% <ø> (ø)	⬆️
featuretools/primitives/api.py	100% <100%> (ø)	⬆️
...retools/primitives/standard/transform_primitive.py	100% <100%> (ø)	⬆️
...ools/primitives/standard/aggregation_primitives.py	94.82% <100%> (+0.13%)	⬆️

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[kmax12]

LGTM