Uses metadata.db to find links targetting a particular page / tag, and
output in one of tsv, vimwiki list, vim location list, or fulltext.

Can be restricted to links from pages with dates, which makes it usable
for pulling logs.

Fully replaces the former:

  - notes-links-for, of which it's a superset

  - notes-logs-by-link

  - notes-logs-for which was used for fulltext of a log, and wrapped
    notes-links-for

This commit pulls into vim config as well.

This spits out links in VimWiki syntax, which is more readable and useful
in context than just a raw list of grep results.

(For the grep results, I could always use `gf` to jump to the file, but
it didn't then get added to the VimWiki location list, so the back command
wouldn't work.)

This also uses an sqlite query against an already-built table instead of
pawing through a bunch of files on disk, which is probably faster.

This is incredibly specific to my vimwiki setup at the moment.  It'll
probably expand to include other things, but I decided to just jam it into
one nice generic top-level namespace for now.