Add ability to customize how attributes are emitted.

[bolivier]

I have a use case that's somewhat difficult to implement without a couple modifications to huff. #29 is an issue I ran into while trying to do the same thing.

I am trying to customize how certain attributes are emitted. Essentially I want to (in my own application) keep a mapping of keywords to strings, and do a swap when those keywords appear in attribute maps.

Previously, I expanded all the hiccup manually and walked the tree replacing those values, but it occurred to me that a mechanism of extension for attributes could be valuable in other contexts.

I have a potential implementation in my fork that I'm using now, but it's somewhat tailored to what I need. There could be a better more generalized solution.