Add another hook to "live encode" format=flow

[wberrier]

based off some code at:

https://emacs.stackexchange.com/questions/19296/retooling-fill-paragraph-to-append-trailing-spaces

[singpolyma]

I'm trying to use this "directly" without the messages-are-flowing package, but when I add the code and wire it in, I either still get normal fill behaviour (no trailing space) or no fill behaviour (if I disable autofill myself). It doesn't look like it depends on the rest of the package in any way -- maybe I'm missing something?