Expose a selection event

[Tyriar]

In order to support features like copy on select an event needs to be exposed which is fired whenever there is a new selection finalized (on mouse up). There is currently a newselection event to support middle click on Linux but this probably needs to be separate.

It's probably best not to pass the selection text with the event but instead allow consumers to call selectionText. This will save the text being calculated when it's not used.

For those interested in doing a PR checkout https://github.com/sourcelair/xterm.js/blob/v3/src/SelectionManager.ts