Add the ability to parse multipart MIME email strings

[settermjd]

Q	A
Documentation	yes
Bugfix	yes
BC Break	no
New Feature	no
RFC	no
QA	no

Description

This change adds the ability to instantiate Message objects from multipart MIME email strings. It's dependent on https://github.com/laminas/laminas-mime/pull/36.

Most of the functionality was already there; there was even a commented out stub that outlined the start of what was required. However, I think because the supporting functionality was lacking in laminas-mime, it was not implemented.

I'm submitting this PR for a couple of reasons. Partly because I'm not aware of another library or package that comes close, because I need the functionality, and because it seems worth finishing the existing implementation.

[settermjd]

I like this, but we need a green build

I'll get it sorted.

[settermjd]

Hey @Slamdunk, at this stage, I've done all I can without https://github.com/laminas/laminas-mime/pull/36 being merged, as this one is heavily dependent on it. Any chance with support to get it reviewed and merged?