Add ability to style suggested action flippers

[corinagum]

Fix #1586
Add the ability to customize the appearance of suggested action flippers.
Note that these options also affect the carousel flippers and scroll to bottom buttons.

   styleOptions: {
     transcriptOverlayButtonBackground: 'magenta',
     transcriptOverlayButtonColor: 'blue'
   }

[coveralls]

Coverage decreased (-0.1%) to 53.131% when pulling b980749 on corinagum:1586 into f68cf3c on Microsoft:master.