Support multipart/alternative, rather than just /mixed, ideally with nesting

When using attachments, swaks will use `multipart/mixed` for the container, and each attachment becomes a sibling in it.

But, this is not always desired.

Sometimes a user may want `multipart/alternative` for the container, or they may want a nested structure, with sub-parts within sub-parts.

Some relevant discussion here: https://stackoverflow.com/questions/3902455/mail-multipart-alternative-vs-multipart-mixed/23853079#23853079

I guess at a minimum, it would be good to be able to specify the type of the top-level multipart container.

A more thorough feature would be to support an arbitrary nested structure of MIME parts.