RFC-3030 (CHUNKING and BINARYMIME)

[perfaram]

This PR implements RFC 3030, which defines the CHUNKING and BINARYMIME extensions.

This implementation is rather conservative in the changes it makes to gen_smtp, in particular regarding the API.

On the server side, by default:

• CHUNKING is enabled, as it is possible to fully abstract away how a message was delivered, from the PoV of a user of this library;
• BINARYMIME is disabled (unless activated in the handle_EHLO callback by setting it to true) because such messages always bypass the newline fixing and may not be UTF8, contrary to what one might reasonably expect when implementing handle_DATA. It is not currently possible to know whether a message was sent with this body type, should this be implemented? Feedback welcome here.

On the client side, by default:

• CHUNKING (i.e., the BDAT command) will not be used, even if possible, for now (I'd like to have feedback first, but otherwise it should not be a problem to use it by default whenever possible);
• BINARYMIME will not be used, unless enabled through the options (so, session-wide -- I think it might make sense to allow setting this per-email? Feedback welcome here).