Allow setting sent_source on a per account basis

[lamarpavel]

In config.yaml the sent_source can be set to a source, allowing to synchronize your sent mail via offlineimap etc with other mail clients or instances of sup running on a different computer.
However, this option is global and if I have more than one account, or several alternates for one account all the mail I send will be saved to the same sent_source (eg. a maildir associated with only one of your accounts) regardless of the account/address I'm sending the mail from.

This could probably be done with a hook somehow, but I feel this is essential enough to implement it as a supported feature.

[kols]

+1

[gauteh]

Related to #255