Added feature: Class Names

[blizzardengle]

I added the ability for users to attach their own class names to buttons. They do this like so:

className: "fa-rotate-90"

or if they would like to add multiple classes to the same button:

className: "fa-rotate-90, custom-class"

This allows users to easily apply some native styles like the fa-rotate-* styles from Font Awesome as well as remove the styling from their toolbar configuration file if they prefer to manage it in their own style sheet(s).

To test my changes add lines 191 - 194 from the lib/flex-tool-bar.coffee file into your package and then paste the following into your toolbar.cson:

{
    type: "button"
    icon: "sitemap"
    iconset: "fa"
    className: "fa-rotate-180"
    tooltip: "This is just an example it does nothing"
}
{
    type: "button"
    icon: "sitemap"
    iconset: "fa"
    tooltip: "This is just an example it does nothing"
}

I have also updated the README to show examples of this new feature.

[cakecatz]

@blizzardengle Thank you 👍