Some enhancements to brackets-automatch-pairs extension

* Introduced _matchStack to handle nested matching pairs and _deletedToken to remember last deleted character. Editor now listens for 'keyEvent' to support this feature.

* Fixed issue where a closing matching character is not deleted when it is exactly the same as the opening character e.g. ",',`

* Removed references to _lastCharacter and _document
* set _enabled to true by default