Introduce a favicon middleware to handle /favicon.ico requests

Hi all,

I don't know if you are interested in a contribution like this. I did it as an exercise to learn a bit about how Nickle works. If it's something that fits with the project's short term goals, great! I modeled it closely after express's serve-favicon middlewhere. The biggest omission is the cache-control header.

I also wasn't sure of the best way to handle errors both at the file io level and for bad requests. Any input there would be appreciated.

Jeff