Allow the mappings to be skipped.

As a plugin, this can easily collide with a user's existing mapping.
Instead, let's introduce a variable to prevent setting up the mappings.

I've been playing with Syntastic and wanted a way to toggle the location list, but your plugin ended up stomping over some existing mappings that I love.  This change allows me to setup my own mappings without stomping on `<Leader>q` and `<Leader>l`.