Extended headers command plugin

This patch will need to be re-rolled WRT the latest dev version, and needs to get to RTBC status before I can commit it.

I haven't forgotten about this, but it needs some work. For instance, presumably the reason these 'missing' headers aren't returned by imap_header is because they vary from message to message. But if the headers can't be known a priori, how can they be mapped in the processor?

I wonder if this might benefit from #1113888: Command arbitrary fields / CCK fields.

This should be re-rolled as a commands plugin if you are still interested. I would suggest copying the existing MailhandlerCommandsHeaders plugin as a starting point.

Also, now that command plugins can be individually configured, it would be almost trivial to write a plugin that allows you define arbitrary headers to be mapped in the processor (basically a combination of the parsing/processing of the current Headers plugin and the configuration options of the Default commands plugin).

By the way, this would be a great little project for anyone new to development and wanting to get some experience with the Ctools and Mailhandler APIs. I'd also be willing to knock this out in exchange for nominal sponsorship... contact me if you are interested.

http://drupalcode.org/project/mailhandler.git/commit/79769f6
http://drupalcode.org/project/mailhandler.git/commit/ecc853c