Reload handler

This is intended to be a unobtrusive addition to werkzeug.serving.run_simple() which adds a reload_handler parameter. This parameter can be used to specify a function that should be called whenever a file watched by the reloader changes. The changed file's full path is passed to the function as the first and only argument.

My personal motivation is to integrate werkzeug's reloader with other compiling tools, such as a CSS compiler I'm currently working on. But I can imagine it to be useful in a wide range of scenarios.