Music metadata

[roipoussiere]

As explained in #578, this adds support for music metadata.

• add function to look for music metadata by looking for tags in code comments;
• update some tunes to add those metadata;
• display title from music metadata in user patterns;
• display title from music metadata in examples page;
• add metadata section in the docs.

[felixroos]

great, thanks so much!

[roipoussiere]

Thanks for the review.

Updates:

• parser now supports multiple values and one-liners (see doc);
• added tests;
• fixed and improved documentation.

[roipoussiere]

I find the syntax // @title Awesome song @by Sam a bit too verbose and disturbing, so I added an alternative syntax using quotes: // "Awesome song" @by Sam.

[roipoussiere]

Ready to merge for me. :)