Add option to not set default keybindings

[maxmahlke]

I see that the default keybindings <leader>ds, <leader>dS, and <leader>DS are set in case the respective functions have not been assigned to another keybinding yet. In my case, I'm defining the keybindings dynamically only for certain filetypes. The default keybindings are therefore still set. It'd be nice to have an option to disable them altogether.

[bergercookie]

Hi there, agree, would be nice to have an option. Please consider making a PR for this :)