Added subaccount support

Hi there!

Thanks for putting together a great bundle. We use it with almost all of out Symfony projects. One thing we missed was support for Mandrill subaccounts. We took the time to add it to the bundle and would love to have this merged in with the bundle for everyone to use.

| Q             | A
| ------------- | ---
| Bug fix?      | No
| New feature?  | Yes
| BC breaks?    | No
| Deprecations? | No
| Tests pass?   | Yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

Please let me know if any changes are needed to make this PR fit perfectly into the current codebase! I'd be happy to change, add or reformat stuff.

Thanks again!