Fix repeated loading clobbering <CR> map chain

Matchem’s careful chaining of existing `<CR>` maps is clobbered by its own `s:Init()` function if the plugin is ever reloaded (which happens once in a while behind the scene when fiddling with `rtp` et al.). Though `s:Init()` could check the existing maps for the presence of its own function, the simpler solution seems to be a guard ensuring the plugin is only ever loaded once.