Add ability to toggle visibility with show/hide functions

For: https://github.com/AmpersandJS/ampersand-dom-bindings/pull/31

I was a bit conflicted on how to implement this, specifically for `dom.show`. I wasn't sure if it should have to be passed the mode as well, or if it should automatically know (maybe with storing the mode in an attribute). I ended up going with passing the mode, so they can be independent of each other. I'm open to either though.