Add multimeasure rests to spec, with example

[adrianholovaty]

This pull request adds multimeasure rests to the spec, along with an example document. This is based on the agreed-upon approach from #284.

The updated docs (as of this pull request) are visible here: https://cdn.githubraw.com/w3c/mnx/multimeasure-rests/docs/

Here's the example document: https://cdn.githubraw.com/w3c/mnx/multimeasure-rests/docs/mnx-reference/examples/multimeasure-rests/

@cecilios and @clnoel — mind reviewing to make sure everything matches our discussion?

[clnoel]

This looks like what we agreed, although I might have added the page and system tags into the part scores in order to clarify the sibling relationship.

However, I am now throwing a wrench in the works. We either need to decide to support the old-style rest format, or decide to put it off until later. We shouldn't forget about it. As recently as last year people were trying to add this to MuseScore.

--Christina

[adrianholovaty]

@clnoel Thanks! I've just created issue #286 to collect discussion/ideas for the old-style format. I don't think the lack of that should block this particular pull request, but I do want to make sure we don't lose track of it.