Allow disabling/overriding the default keymap

[zhimsel]

If a user wants to set they're own key map for the toggle function, the one set by this plugin still exists. This can obviously just be overridden in the user's vimrc, but this is cleaner.

EDIT: just saw this is a dupe of https://github.com/jkramer/vim-checkbox/pull/8; no idea why that was closed