Add optional image for button class

[janbrouwer]

I also updated the example in a bit of a crude manner(didn't update the css for this specific image so it partially blocks the text).

[thomthom]

There appear to be additional changes pushed to this merge request. Is that intended? When you push new changes to a branch set up for a merge request it will append to that request. If you want to keep things separate you'd need a separate branch - which is often done, creating a feature branch.

[janbrouwer]

Sorry, I was indeed intending to create seperate pull requests for these additions(they seem very simple and useful), and cancel the original one(for now, until I find a better solution). I will try to set up separate branches for every feature request...