Add window alert support

• alertBeforeUnload

  • When this props is true, the component will also add onbeforeunload event to the window object that detects the page to popup an alert before you leave without saving data changes.
  • This event fires when user close or reload the current page. The reason is that there is no way to detect close/reload actions except this event.

• alertMessage

  • Custom browser popup message.
  • Note that the text message is only for IE users due to browser limitation.