linkcache.sqlite3: don't expect connections to persist

Supybot runs multithreaded and creating a connection in one thread while
using it another really makes the sqlite module angry.

Signed-off-by: Jeff Mahoney <jeffm@jeffreymahoney.com>
linkcache: add minimum_length and allow_interpolation options

The interpolation aspect of linkcache may be annoying at times, so let's
make it configurable. Also, we can test for a minimum url length to
exceed before printing a short link.

Signed-off-by: Jeff Mahoney <jeffm@jeffreymahoney.com>