Added flipperBlurFocus prop

[tdurnford]

Per microsoft/BotFramework-WebChat#1673

This PR adds the flipperBlurFocus prop to BasicFilm and the blurFocus prop to Flipper components. When flipperBlurFocus is false, which is the default value and maintains the current behavior, the flipper will keep focus on click; however, when it's true, the flipper will blur on click. Regardless of the truthiness of flipperBlurFocus, the flipper will keep focus when the user is interacting with react-film through the keyboard.

<BasicFilm height={ 316 } flipperBlurFocus={true} key={ `${ key }-a` }>
  <img alt="Cat 01" src="image/01.jpg" />
  <img alt="Cat 02" src="image/02.jpg" />
  <img alt="Cat 03" src="image/03.jpg" />
  ....
</BasicFilm>