[feature] support for button 'pressed' state

[morenoh149]

I need buttons that have boolean state (on|off). After doing research the :active state is unsuitable for this http://stackoverflow.com/questions/12960068/keep-css-active-state-after-click-with-jquery more preferable is to add an .active class. Then in jquery you can do $('button').toggleClass('active') upon click. I just tried this out on my local build, all I had to do in the css is to replace every instance of :active to .active. I propose this library drop support for :active and switch over to using a class.

[alexwolfe]

:active is a pseudo class that happens when a user is clicking on a button. We won't be removing this state but rather adding a .active or .is-active class to the button so that you can use jquery to toggle the state programatically.

[alexwolfe]

@morenoh149 I just pushed this change to the repo, it will be avaialble on the builder soon.

To use it simply add .is-active or .active class to any button. Enjoy!