word-wrap and wrapping before window-width (was: bug#19462: shr: use wrap-prefix when possible, instead of filling the text)

[Stefan Monnier]

>       The only feature that I’m aware to be missing is the actual
>       support for Emacs native text wrapping (as in: the word-wrap
>       variable and wrap-prefix text property) in SHR.

Reminds me that while I opposed adding a `wrap-column' option for those
people who want to do word-wrap without using the whole window width,
I would actually welcome a new text-property which lets the wrap-column
be set locally to a different value (ideally, this should also allow
specifying that some part of the text should be word-wrapped while
others should be truncated).