Add support for Reply-To field in mail_to helper

Although not explicitly listed as allowed in the [RFC for the 'mailto' URI scheme](http://www.ietf.org/rfc/rfc6068.txt), it is also not explicitly defined as disallowed. In that respect, this new "reply-to" field seems to be in the same boat as the existing "bcc" and "cc" fields.

This is verified to work as expected in Apple Mail on Yosemite. I have not checked in other clients.