Options & Callbacks

Great plugin! Very simple and easy to use, but I needed a little more flexibility and customization. I've added the following options & callbacks:
```javascript
targetClass: 'responsive-tabs', // Base class to look for, also determines all element & state classes (__heading, etc)
defaultSpeed: 0, // Default speed of the tab toggle. Set to 0 for instant switch
responsiveSpeed: 150, // Default speed of the responsive accordion slideToggle. Set to 0 for instant switch
onTabShow: function(){},// Callback function when a tab is shown
onTabHide: function(){} // Callback function when a tab is hidden
```

Feel free to adapt this based on your vision for the plugin.