emacs: update coolj-line-prefix-regexp to make space optional

Jani reported that lines prefixed with '>' (as opposed to '> ') are
not highlighted properly [1]. David E responded with a updated regex
[2]. This change implements David E's suggestion.

[1]: id:87a8b5pcky.fsf@nikula.org
[2]: id:m2pok1e3gv.fsf@dme.org