Easier way for adding custom operations

[RogerTangos]

Trane needs an easier way to add customize operations. Currently, external plugin operations are not allowed. The bottleneck is we need to maintain a list of operations so that we can save, load, and iterate over operations. It's not easy to add an external operation into operation list.

It may be helpful to emulate how FeatureTools allows feature engineering primitives to be passed in.