pre-processors for markdown files

[Jacoby6000]

It's be nice if we could provide an SBT setting which is essentially a List[FileContents => FileContents] where each function represents a preprocessor before the file is sent to tut. This would allow people to write pre-parsers which could look for and automatically link issue numbers, link source code when certain keywords or patterns are found, reference github users when they pop up in a contributors file, and more.

[Jacoby6000]

Thinking on this more, tut is just another pre-processor. Introducing this support might lead to a better integration with tut.