allow plugins to add express middlewares

enable plugins to insert middelware _after_ a file is processed and _before_ it's sent to the browser

use case: javoire/orangejuice-livereload#1
lingon-livereload implementation: javoire/orangejuice-livereload#8

I think what's important here is to generalize this use case to come up with a reasonable solution that would be useful for all plugins :)

ideas and thoughts are welcome!