There is a number of ways to achieve this. Probably one of the simplest – you can set left arrow symbol code, i.e. ← as “Button label” in our widget’s settings.
Please let me know if that’s not good enough, and I’ll come up with other solutions.
If that doesn’t help – please update the plugin to the latest v1.2.4 that we’ve just released. There I’ve added special fa attribute to the shortcode – now you can use Font Awesome like this: