cli: add top level `tag` listing support

[sondove]

Right now we support listing the tags for a plot, but not all plots for a tag.

If the user uses -{p,m,g,d} without a specific plot then we should suppor -t tag_id to list tags of only that id.

For example novem -p -t fav should list all the users plots tagged with fav

We should consider a top-level novem -t fav that would list all favs regardless of type (including everything from plots to jobs)