Support nested message sections

[Tim203]

This PR adds support for nested message sections like:

public interface RootSection {
    HelloSection hello();

    @MessageSection("hello")
    interface HelloSection {
        @Message("world")
        String world(@Receiver User user);
    }
}

and

@MessageSection("hello")
public interface HelloSection {
    @Message("world")
    WorldSection world();

    @MessageSection(delimiter='-')
    interface WorldSection {
        @Message("moonshine")
        String moonshine(@Receiver User user);
    }
}

(first one results in key 'hello.world', second one 'hello.world-moonshine')

Please let me know if there are some changes you'd like to see.

[broccolai]

LGTM in general, I'll touch up a few things and merge it. Thank you!

Sorry it took so long feels like only yesterday I saw your ping haha