Allow custom timeout for notifos messages

Two small changes to notifos.

The first adds a custom timeout argument to notifos/set-msg! and adds a one-argument method to the function to avoid arity errors.

The second refactors the related msg* by making opts into an actual optional argument; no functional changes, but it eliminates a compiler warning.

Of course, feel free to cherry-pick the feature commit if you don't care for the warning pedantery.