Allows user to opt-out of key remaps

This patch allows a user to opt out of vim-sensible's key remappings by adding the line `let g:sensible_suppress_maps` in their `.vimrc` file, instead of requiring them to create an "after" script to undo the mappings.

(This is a pretty trivial change, and maybe a different variable name would be preferable in order to match your preferred naming conventions.  There wasn't an example to draw from inside this plugin, so I just used the first name that came to mind.  Please feel free to modify to fit your preferences.)