Add option g:indent_guides_default_mapping

Hi.

Thanks for writing this plugin! :)

I just made this little change to not provide any mapping by default, since the checks that were in place already are not enough. If one loads plugins before mappings the first check is not useful, and if one maps something to <leader>i, both <leader>i and <leader>ig work, but the first with an annoying delay.

Thank you.