Plugin system for `Diff*` engine#18

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ambsw-technology wants to merge 3 commits intoruntheops:masterfrom

ambsw:engine-plugins

Commits on Apr 29, 2019

refactor to add documentation, clarify variable names, add test cases, and better encapsulate behaviors (among other things to simplify testing)
claytondaley
committed

Commits on Apr 30, 2019

Add plugin system for `Diff*` engine (relocated to `engine`) and simplified/standardized the interface (i.e. `configure`, `merge`, `plan`, and `__init__`)
claytondaley
committed
fix bugs
claytondaley
committed