Store tag descriptions in inst/tags.yml

[hadley]

Fixes #1375. Fixes #1165.

[kevinushey]

nit: maybe consider storing the tags in inst/roxygen/tags.yml? So that R packages could collect any roxygen-related metadata into that directory (and avoid a theoretical collision with something else at inst/tags.yml)

[hadley]

I could make it roxyen-tags.yml, but I'm not sure I want to imply a whole directly worth of installed/installable roxygen2 files.

[kevinushey]

That makes sense to me.

[hadley]

I'm going to merge this because I think it's a good start and I want to get a roxygen2 release out before conf. We can iterate more once y'all start working on for the IDE.