Replace class-level @FlagDesc with @InjectFlags #10

The interface will mark a class as subject for Flag value injection during classpath scanning. It will also provide hints to the flag injector, like the flag variable prefix, intended to allow for more concise code.

public @interface InjectFlags {
  String prefix() default "";
}

The prefix hint allows for concise naming convention and resolves collisions with instance variables:

@InjectFlags(prefix = "flag_")
class MyThing {
    @FlagDesc("This tells me what to '--do'...")
    static Flag<String> flag_do = Flags.create("Rest!");

    final String do;

    MyThing() {
        this.do = flag_do.get();
    }
}