Make the "vendors" variable overridable

This simple fix will allow developers to front-load the `vendors` variable with a different value and prevent `bootstrap-stylus` from overriding the one that was previously set.

When I updated to the latest version of `bootstrap-stylus` in one of my projects, all of my keyframe animations broke, and I traced it down to the fact that `vendors` is no longer set to the stylus default of `moz webkit o ms official`.