[Mime] Allow custom encoders for parts

[KDederichs]

Description

Add the ability to add custom encoders to Parts (DataPart/TextPart).
Currently you already have a list of registered encoders but it's private static (and has no values as far as I can tell).
In addition to that you also hard code the 3 encodings and throw errors when trying to pass another one.

It would be good if you could just add custom encoders (via constructor or addEncoder method)

Example

The reason I want to use this is I'm trying to implement Soap Attachments (SwA, https://en.wikipedia.org/wiki/SOAP_with_Attachments ), which is basically SOAP encapsulated with Multipart/related Mime parts, so using a Raw Email format is the best starting point for this I think.

That specific protocol supports binary encoding but sadly I can't easily add an encoder for this.