Add the --allow_different_domains flag -- fixes #36

This new flag ignores the error raised when you change the list of SANs. That
error is generally useful, because it alerts you of a mistake you might have
made, for example forgetting to add a domain, but it can be annoying if you
generate the list of domains from a script, and that list changes over time.

A possible use case for this is a list of domains managed by a configuration
manager, and a script which generates simp_le's command line arguments and
calls it. In that case, you're sure the list is correct, and maybe you're just
adding a domain.

Since in that environments a manual fix can be annoying, the newly introduced
flag disables that behavior if provided. This way, the error is kept for the
common use cases, but if someone needs to ignore it he can.