[CSL 1.1] Collapse `<names>` variables

[bwiernik]

I think this may be part of the draft CSL 1.1 already, but noting here while it's on my mind.

For multimedia items, APA and other styles call for multiple variables to be entered in one string. For example:

Doe, J. (Writer), & Smith, J. (Director)

Additions to CSL 1.1:

1. collapse and collapse-delimiter arguments on <names> to indicate that the variables should be combined and rendered as one list rather than as multiple sequential lists. When collapse='true', labels should be printed after each name, rather than at the end of the list.
2. When collapse='true', identical names should be combined into one (e.g., Doe, J. (Writer & Director)). This might need another argument combine-names or similar and/or an argument to control the "and" delimiter used (e.g., text, ampersand, /)