Add support for Mandrill subaccounts

[janhenckens]

Mandrill has the concept of subaccounts to separate out different types of users/clients/projects. The accord/mandrill-swiftmailer this plugin uses supports subaccounts so I thought I'd add a setting for them.

(Not sure how widely used this is or if this should be in the general Mandrill plugin, if not I'll happily make a separate plugin with this in it)

Cheers Jan

[janhenckens]

Updated! :)

[brandonkelly]

Thanks!