Custom primitive creation functions

[rwedge]

Added two functions, make_trans_primitive and make_agg_primitive. Given a function to apply to input features, list of allowed input feature types, and variable type of the resulting feature, these functions will create a new primitive that can be used with DFS.

[kmax12]

do you need to add cloudpickle to requirements?

[rwedge]

added

[codecov-io]

Codecov Report

Merging #13 into master will increase coverage by 0.42%.
The diff coverage is 95.19%.

@@            Coverage Diff             @@
##           master      #13      +/-   ##
==========================================
+ Coverage   86.44%   86.86%   +0.42%     
==========================================
  Files          73       73              
  Lines        6549     6778     +229     
==========================================
+ Hits         5661     5888     +227     
- Misses        888      890       +2

Impacted Files	Coverage Δ
featuretools/demo/mock_customer.py	14.7% <ø> (ø)	⬆️
featuretools/utils/pickle_utils.py	75.86% <100%> (ø)	⬆️
featuretools/primitives/aggregation_primitives.py	92.1% <100%> (+0.85%)	⬆️
...ols/tests/feature_function_tests/test_agg_feats.py	98.35% <100%> (+0.75%)	⬆️
featuretools/primitives/utils.py	93.15% <100%> (+1.48%)	⬆️
featuretools/primitives/primitive_base.py	76.94% <79.16%> (ø)	⬆️
featuretools/synthesis/deep_feature_synthesis.py	92.09% <88.88%> (+0.13%)	⬆️
...turetools/primitives/aggregation_primitive_base.py	95.45% <92.59%> (-4.55%)	⬇️
featuretools/primitives/transform_primitive.py	96.78% <94.44%> (-0.51%)	⬇️
.../feature_function_tests/test_transform_features.py	85.15% <95.65%> (+0.89%)	⬆️
... and 3 more

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