[Exim] Moving rewrite rules out of the main configuration fi…

Hi,

We're looking at using Exim is a virtual hosting environment. Each
customer will get their own instance of Exim in their chroot()ed
environment. The main configuration file will be root-owned and not
modifiable by them, this is to avoid them setting up open relays and the
like. This will reference files like sendmail.cw and virtusertable
(silly names for compatibility with previous system) which customers can
modify to configure their mail.

We'd like to let customers create their own rewrite rules. However,
there doesn't seem to be a way of including a list of rules from a file.
I know an individual rule can go lookup data in a file, but that's not
what we need.

Thanks,

Paul