Handle Epistolary Novels #137

We could build a method that takes in a Document, detects whether or not it's an epistolary novel, and then breaks the document up into a dictionary of letters (or a list of Letter objects?). We'll want to programmatically detect the writer of each letter and include that in the metadata.

Ideally, we can programmatically determine metadata for each letter - writer, date, recipient, and so on. That's going to be tricky, but maybe possible. If we combine this functionality with our hypothetical named entity recognition module (to get a character list) and a ML-based gender guesser for each character, we can do some classy stuff.