Source regardless of g:rainbow_active existence

I think it's kind of unexpected to source the plugin only when `g:rainbow_active` is defined.

If you want to use it sporadically through the `:RainbowToggle` command, having to set `g:rainbow_active` to 0 doesn't really follow the principle of least astonishment.
`!(exists('g:rainbow_active') || exists('g:rainbow_conf'))` <==> `(!exists('g:rainbow_active') && !exists('g:rainbow_conf'))`
it says that, if none of global variables about rainbow defined. this plugin should be skipped.