#266 - Added automatic paired characters

Implemented paired characters, as described in issue #266. For the characters (), [], {}, '', "", <>:

1. When inserting the opening char, the closing char gets added automatically.
2. Deletion of the closing brace is automatic when backspacing the opening char, and the close is immediately to the right.
3. Overtyping of the closing brace is also implemented.
4. Highlighting of matching pairs has also been added.