New option: vimrplugin_user_maps_only

The plugin creates a lot of key mappings.  When turned off (default
value) the plugin continues to create the mappings.  When turned on,
then the only mappings that are created are those explicitly set by the
user.
Update help doc for vimrplugin_user_maps_only
Merge pull request #114 from mdlerch/selected_maps

Selected maps