Improved code layout.
Moved 'accounts' plugin to a new name, 'mailboxes'.

Removed the config in anticipation of a new format.
Documentation describing the new config layout for 'mailboxes'.
Documentation for the 'aliases' plugin.
Documentation in anticipation of new 'queue_outbound.accounts' plugin.

This plugin will provide authorisation for outgoing email, so users can only send from particular email addresses, rather than anything they like.
New plugin, 'queue_outbound.accounts' added. Tested and seems to work.
Fixed a logging issue and the cyclic alias checker.

I think the cyclic check could be better if we check the original email against the bottom one, as well as each one along the way. This would fix both A->B->A as well as A->B->C->A, and it'd work for a horrible mix of the two like A->B->B->A.
Fixed the config['user'] bug.

Also moved all the returning to the bottom of the function so the flow of code is easier to follow.
Wrote a proper README, will add some licensing soon.

Also updated .gitignore.