Extra context: API + CLI

Fixes #12
Synthesizes parts of: #75, #100, #116, #161 and #199.
Thanks @fcurella, @aventurella, @emonty, @schacki, @ryanolson!
Let's break the following into 1-2 new sections at the same level as "Calling Cookiecutter Functions From Python" -- might be too much for a reader to grasp all at once