Add Support for Sending HTML and Text Bodies in Single Email

[cristosal]

Hello,

First of all I want to thank you for creating and maintaining this library, I have used it now in production for quite some time and have enjoyed using it thoroughly.

Currently the email package only supports sending either text or html messages, with the text message being chosen when both fields are present.

I have read from various sources that it is considered best practice to include both text and html message bodies in the email so that the email client gets to choose which format to display to the user. I believe this is a fairly simple change in the codebase and am more than happy to contribute myself.

Please let me know how to proceed on this!
Thanks