Rewrote the bracketcompletion plugin for more intuitive behaviour

[thegame1999]

Features:

• Auto insert close brackets at appropriate times
• Enable selecting text and hitting an open bracket to wrap the text
• Manually closing an auto inserted bracket works as expected (overwrites)
• Backspacing a new open bracket works as expected (removes counterpart)
• Navigation with the arrow keys resets the state thus confirming the auto inserted brackets
• Enter key handles auto indented blocks like modern IDEs

Addresses feature request (#660) and issues raised (#700).