Addition of BodyPart::Raw

[mdsimmo]

I'd like to be able to write a custom encoding type, as allowed for by RFC 2045 p.15.

This body type would operate exactly like BodyType::Text/Binary, but not add any Content-Transfer-Encoding headers, and directly write a raw [u8] to the output.

[mdecimus]

You can add use a Binary part and add the headers you need manually. No changes are needed in the code to support this.

[mdecimus]

This is now possible on version 0.3.1, available on the repo.