change PHP comment style to // instead of /* */

Since moving from NerdCommenter to Vim Commentary, I haven't liked the
way PHP is commented with /* */ (Vim Commentary) instead of //
(NerdCommenter).

This setting in my .vimrc overrides this behavior in Vim Commentary to
use //
add <leader>c<SPACE> mapping for vim commentary

Add normal mode <leader>c<SPACE> mapping for commentary. This makes the
behavior more like NerdCommenter