Override for config directory.

Override for config directory.

This provides a crude means of managing multiple accounts. At the
moment my only use case for this is logging in to a test server so
that I can test the fine points of posting without making a fool of
myself on the live fediverse. So I don't need to run the accounts in
parallel in a user-friendly way, and I can easily just pass a weird
command-line option when I'm doing testing.