Text direction icon should flip the arrow when pressed

[benbro]

[Describe the issue]
The text direction icon should have an arrow on the left and pointing to the left.
When clicked, the arrow should flip to the right side and point right.
Please see the icons in the attached image.

Steps for Reproduction

1. Visit http://beta.quilljs.com/docs/formats/
2. Click on the text direction button.

Expected behavior: [expected]
The arrow should start pointing to the left and flip to the right when pressed.

Actual behavior: [actual]
The arrow always point to the right.

Platforms: [ex. Chrome 48 on Mac 10.11]
Chrome 50 on Windows 7

Version: [version]
1.0-Beta.0