To allow adding neosays configuration to a dotfiles repo, it's
beneficial to allow providing a command to retrieve the access token
via a command (for example from a password manager).
If an access token is set while a command is set as well, the token is
overwritten in favour of the securely retrieved one.
Fixes: https://github.com/donuts-are-good/neosay/issues/1
Signed-off-by: Moritz Poldrack <git@moritz.sh>