add onBeforeSelect callback

[jnelson]

Adds an onBeforeSelect callback to allow intercepting a selection. Ensures
access to both the prospective new selection and the current selection.

This is a preliminary offer for #1071 - let me know if you'd like any additional tests, style changes, etc.

Features:

• provides a way to access both the incoming selection and the current selection, e.g.:
  • $scope.confirmMatch = function(currentSelection, incomingSelection) { ...
  • <ui-select on-before-select="confirmMatch($select.selected, $item)" ...
• defaults to completing the selection
• allows aborting the selection synchronously by returning falsy
• allows fulfilling or aborting the selection asynchronously by returning a $q promise

[jnelson]

Updated the features explanation above to match code at this point.

[entrity]

👍

[wesleycho]

This needs squashing and rebasing.