Added callbacks token un/selection events

[rapito]

Token Selection Change Event

For ease of access and rapid development, I've added a callback feature to be triggered on selection change event of tokens.

• Added onSelectedToken callback function to be passed as a setting: Called when any token is selected.
• Added onUnselectedToken callback function to be passed as a setting: Called when any previously selected token loses focus.
• Added Demo to demo.html.
• Method Paramater signature for both callbacks is (element, tokenData):
  • element: DOMElement of the un/selected token.
  • tokenData: Data Object bound to un/selected token.