Fix addMarker and addRegion
Fix addMark(), addRegion() and startEdit()

* Add g:multiedit_mark_character (customize the marker)
* Remove bindKey() (unused)
* Set g:multiedit_auto_reset default to 1
* startEdit() now places the cursor in the right place
* MultiSelectionsFirst highlight laid out (not implemented!)
Implement MultiEditAddRegion highlight
Allow for custom highlights