Card-reveal with more flexibilities

I was trying to use the card reveal feature in one of my personal projects.
I had a button on a `.card`, and that button always pulled up the hidden card everytime I clicked it because of the way the click event was binded to the card in the original `js/cards.js`

I refactored `js/cards.js` based on the original function written by @acburst and made an extended Jquery function. The new `reveal()` function gives Materialize users more control over the whole feature now.

Updated the documentation, too!