Initial stub for an \annotate function

In its current form the function simply offers a signature and colors an affected grob magenta.

The idea behind it is to annotate music directly in the music input file.
One will be able to enter a set of different preset annotation types and also arbitrary ones. The presets should have some predefined behaviour while the arbitrary ones just get output, leaving it to the user to make use of them.

The idea is to (optionally)

- write messsages to the command line
- write entries to an external file (like latex's toc files)
- highlight (color) grobs
- (maybe) print annotations on top of the score (once it will be able to "print" to layers.

Please comment on the signature, add possible use cases, ideas for recognized options.

See [this file](https://bitbucket.org/beautifulscores/das-trunkne-lied/src/bb4e5ca2256c900c140480462054854b631ff23e/parts/violoncello/05.ily?at=vc-modify-compile-segment) as a (preliminary) real-world example file.