Allow arrows to be placed in toolbar default buttons

[jaimestuardo]

If I want to add arrow icons to the next and previouse buttons, I did this in initialization:

            `lang: {
                    next: 'Siguiente <i class="fa fa-arrow-right" aria-hidden="true"></i>',
                    previous: '<i class="fa fa-arrow-left" aria-hidden="true"></i> Anterior'
                },`

However, that does not work.

This is how they are shown:

Regards
Jaime