added possibility to set sender address

As stated in [Email::Sender::Manual::QuickStart](https://metacpan.org/pod/Email::Sender::Manual::QuickStart#envelope-information), it is sometimes necessary to be able to set the e-mail sender (e.g. MAIL FROM in the envelope) to a different address than the address given in the From header line.

This patch enables you to do this by passing a "sender" key to Dancer::Plugin::Email.
I am not sure, if this is the most reasonable implementation, though – suggestions welcome!