options to show both HTML and plain text versions

I have noticed that I sometimes get mail with HTML and plain text parts that have different content.

I think it would be useful to be able to enable showing both HTML and plain text parts, so that I can more easily detect the mails with these differences, which could be significant in some contexts.

Usually the mail's order of these parts will be fine, but some folks might want to reorder plain text parts before HTML parts or vice-versa.

Also, I note that there is an option to show the HTML part as an attachment, but there isn't such an option for showing the plain text part as an attachment. This might be desirable for some people.