sp-last-wrapped-region now uses markers instead of integers to mark the boundaries. This allows for nicer interplay with other packages that might add/remove text and so alter the values, such as multiple-cursors or "electric" modes.
Added support for multiple-cursors. Each cursor tracks its own version of some variables used in wrapping/insertion.
Call fallback function if the insert action is not carried out. This way we can use the key for wrapping and have its old behaviour on insertion.