[enhancement] Add SwiftmailerBundle configuration bridge

[vincentchalamon]

As this bundle is dependent of Swiftmailer, it should be interesting to bridge its configuration through swiftmailer (in a bundle use), like:

swiftmailer:
    # ...
    mandrill:
        transport: accord_mandrill
        api_key: %mandrill_api_key%
        async: %mandrill_async%
        subaccount: %mandrill_async%

(Also rename transport to mandrill) @Rattler3