Add support for the multipart/alternative on publisher and content-type dispatcher on the subscriber

[abogani]

The multipart/alternative subtype indicates that each part is an "alternative" version of the same content, each in a different format denoted by its "Content-Type" header.

Supporting this mime-type in the publisher and make nchan outputs the matching content type (specified by users' "Content-Type" header) in the subscriber (without the burden to convert something) could be useful in my opinion.