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.

[matthiasbeyer]

This plays a bit into what I described here.

The watching functionality is certainly something that is important, but this crate rather focuses on the layering and parsing of configuration, not on the workflow how to handle the configuration.
Such functionality should be implemented outside of the crate, there are other crates that already solve that issue, I guess.