Make Plugins::Runner support attachments from RequestPlugins
Currently a RequestPlugin can't return anything but a string to be sent back to the sender. The need to send attachments is currently solved by sending an email in the plugin and then exit — that's no good. And sending attachments would make sense in other plugins, too.
We have to be aware that (currently) the Plugins::Runner runs ListPlugins, too.
This depends on #171 (closed).