We could definitely do this, but my thinking here was that providing a `-encrypt` might become brittle if the keys are updated at any point. As an example, if I have a script that starts serf with `serf agent -keyring-file /tmp/ring.json -encrypt xxxxxxxxxx`, then each time that Serf starts, the primary key will again be set to `xxxxxxxxxx`, even if a newer key had been set/broadcasted from a member at some other point and saved into the `-keyring-file`.

The `-keyring-file` in its current form is intended to work almost exactly like the `-tags-file`.