Allow sending of any combination of text and html

[TimQuelch]

• Add a new property to messages: format which can take a value of 'text', 'html', or 'both'. 'html' is similar to the old default behaviour, 'both' is similar to the old functionality when org-msg-text-plain-alternative was non-nil, and 'text' sends only text/plain and no 'html'
• Add a custom option to configure the default value of this property
• Refactor org-msg-prepare-to-send to handle the new functionality
• Also refactors org-msg-mml-into-multipart-related so that MIME structures are as close as possible to when recursive mime is present.

Creating this PR now that it is in a mostly working state

My knowledge of mml and MIME are not very deep, so please let me know if I have made any mistakes with the refactoring of org-msg-prepare-to-send or with org-msg-mml-into-multipart-related.

This patch should work with #69 to allow for much greater functionality and configurability with sending text email while still remaining in org-msg

TODO

• Add command to cycle through different format properties for easy switching between text, html, and both
• ~Test when mml-expand-all-html-into-multipart-related is present~ DONE, seems to work

[TimQuelch]

I would prefer that you squash patches fixing this patch into a unique patch. It has not been merged in an official branch yet so we do not need to remember that you originally had a typo.

Yep I was planning to after it was ready to merge.

indent the lines in your commit message

Do you mean in the hyphenated list?

[TimQuelch]

I've modified it to use the a configurable list that should be a little bit more extensible. Let me know if you have any thoughts

[jeremy-compostella]

Do you mind squashing patches so that I can perform a review?

[TimQuelch]

I think many of the indentation errors are because there is a very large mix of tabs and spaces throughout the file. The style of indentation changes line to line.

I'll squash after we resolve all comments

[TimQuelch]

I think it should be ready to merge.

I haven't fixed indentation because of the mix of tabs/spaces throughout the file. I think he best way to approach that would be a separate PR which fixes indentation for the whole file. Also including a .dir-locals which sets indent-tabs-mode to the desired value

[jeremy-compostella]

Thanks for your contribution. I really appreciate that you took the time to take into my comments and improve your implementation.

[TimQuelch]

No worries, thanks for accepting the PR

[arvindsv]

FYI: I mentioned a potential surprise due to this change, which might be useful to others looking at this PR at a later time: https://github.com/hlissner/doom-emacs/pull/4295#issuecomment-737993746