encode: wrap long lines

[oliverpool]

Implements rfc6350#section-3.2

Long logical lines of text can be split into a multiple-physical-line representation using the following folding technique. Content lines SHOULD be folded to a maximum width of 75 octets, excluding the line break. Multi-octet characters MUST remain contiguous.

As draft because it is incompatible with #35 (opened so that you can have a look, will rebase after the merge of #35)

[emersion]

I'd prefer to not implement this feature. It doesn't serve any actual purpose, and from my experience with go-message, is very difficult to get right.

[oliverpool]

Fair enough!

Thanks for the feedback ❤️