Provide access to MIME boundaries #127
Description
It would be very useful if each MessagePart would contain a boundary member, showing which MIME boundary belongs to that part.
The boundary values used in a MIME structure are important for forensic and anti-spam reasons (as they vary from client to client).
It would also be useful in cases where one might want to inform the user about a specific subpart. Such as being able to (syntax) highlight an entire subpart based on a boundary name.
For ergonomics should be directly possible to find a MessagePart given a value of a boundary attribute (from a header like Content-Type).