Add 'icon' prop to buttons

[afilp]

We had the need to customize the icon shown on the ShowButton.

This commit changes just the ShowButton, so that you comment whether you like this change or not.

If you agree with this, let me know so that we also update the rest of the buttons located inside the button folder.

Thanks.

[fzaninotto]

Love it! Would you mind going a bit further and do the same for other buttons?

[afilp]

Sure! That was the idea! First to see if you want it, then take the time to do the rest. I will do this most likely during the weekend.

Thanks!

[afilp]

Added 2nd commit for the rest of the buttons.

Please note that in some "class"-using buttons I moved the propTypes and defaultProps as static. This was done for consistency: Some classes used the static approach, others the named declaration at the end, now all of them use the same logic, the static one. If you prefer the other approach just let me know to do the opposite.

[djhi]

Thanks!