nullmailer: add `remotesFile` option

The current `remotes` option is a string option containing nullmailer remote
definitions. However, those definitions may contain secret credentials and
should therefore not be put world-readable in the nix store.

I added a `remotesFile` option, which allows to specify a path to the remotes
definition file instead. This way, the definitions can be kept outside of the
nix store with more secure file permissions.