Introduce write method to handle value output

[davekiss]

Description

I found myself needing to access the value that was being written to the time tooltip and the only way to do this was to overwrite the entire update method just to set the time content to the TimeTooltip state. This method would allow for overriding a single line of code if the DOM mutations aren't preferred.

Requirements Checklist

• Feature implemented / Bug fixed
• If necessary, more likely in a feature request than a bug fix
  • Change has been verified in an actual browser (Chome, Firefox, IE)
  • Unit Tests updated or fixed
  • Docs/guides updated
  • Example created (starter template on JSBin)
• Reviewed by Two Core Contributors