convert newlines in replacements to be line breaks in the generated docx

This addresses #2

@paulcsmith this is definitely not ready for merging, but I wanted to just take a quick pass at the problem to see what would be involved with making it possible.

The gist of it is that instead of just replacing the contents of individual text nodes you actually have to create a new list of nodes (multiple text nodes separated by linebreak nodes) and then inject that new set of nodes into the document before removing the original node.

The difficult thing right now is that passing in the option to convert newlines happens in DocxTemplater, but the actual replacing of xml values happens way down in the NodesToFix class. So we will need to think about how to structure the the interaction between the code that is walking the xml trees and the code that knows what types of replacements it is supposed to perform.