Allow users to disable default key mappings

It would be nice if users could disable the default `<leader>ru` key mapping, in case they want to set their own, or if they don't want to have any key mappings at all, and trigger by issuing the command manually.

This also is important if someone already uses the combination `<leader>ru` already for some other purpose, avoiding the conflict. The plugin still behaves the same if the user takes no action to explicitly disable the default mappings.