Some functionality I introduced for wrangling text properties in
erc-match.el was shoddily done and not very usable from the perspective
of another module building on erc-match as a foundation. The attached
patch attempts to correct that. It also ditches "erc-" namespacing for
invisibility spec members because the potential for collision seems
pretty low given that we're only talking ERC buffers. (Hopefully, I'm
not missing any downsides here.)