Add a @Config-like annotation

[ljacqu]

Goes hand in hand with #44 (kind of), I'd like to create an annotation that can be put on other annotations to signal that it is a "Config" annotation, i.e. that it has one value associated to it, and that if it was not provided beforehand the injector knows it needs to throw (and it shouldn't try to instantiate the class).

Besides allowing the injector to fail faster and with a more specific error message, it offers the possibility to keep track of all such "config" annotations by checking for the usages of @Config in one's project.

@Config is probably a wrong name for it since this behavior is not necessarily limited to some "config" values.

[ljacqu]

Maybe name it Identifier?