Custom name for prevButton and nextButton

[ghost]

Hi !
The library is really good and match our expectations, except for a little point. I want to specify the name of the previous and next buttons for translation but it's not applied.

Even if i add these options to each step, the buttons' names don't change:

            'nextButton': {text: tutorial_next_btn_translate},
            'prevButton': {text: tutorial_prev_btn_translate},

After few tests I noticed that we can specify the name and class for the skipButton and only the class for the nextButton. The name for the nextButton and the name and class for the prevButton aren't applied.

[ThemisCol]

#147 (comment)