Provide an annotation function for activities' completing-read

[jdtsmith]

This is what marginalia uses to add rich info annotations. I could imagine annotations describing #windows, #files, #buffer, last modified file date, etc. And of course a sort-function which puts activities with most-recently-modified files first.

See the docs for completing-read for info on metadata category.

[alphapapa]

Yeah, and we'll probably need to ask @minad and @oantolin to chime in as well.

[alphapapa]

Since this may need coordination with Embark (or maybe not, I'm not sure), I'm deferring til 0.6.

[jdtsmith]

Marginalia actually recommends against depending on it:

If you add your own completion commands to your package we recommend to specify an annotation-function or an affixation-function, avoiding the Marginalia dependency this way.

See #83 for a draft attempt.