Custom Indicators

[DaveSkender]

Fairly soon, I'm going to provide an example of how to extend this library to add your own indicators (see #698). The reason I want to start this discussion is because the library itself can only take you so far into the universe of indicators. It makes sense to help users, both in features and in documentation, on their journey of creating personalized indicator algorithms.

Thoughts and feedback as I get into this work are appreciated.