Re-thinking: Config "watching"

[matthiasbeyer]

The crate should provide an easy way to implement config "watching" (as in inotify). A user might want to make its application able to react to a changing configuration file, while their app runs.

First of all, this should of course be feature-gated as necessary. Secondly, I like to believe that this can be implemented in a way that we do not provide the "notify backend" at all, but give the user the opportunity to use interfaces we provide, to build such behavior.