Debounce resize events to improve performance

When using more than a couple fluid placeholders at a time, the performance of resizing the browser window can degrade significantly.

To fix this I added a debounce function to the resize event. Basically it only calls ```resizable_update()``` if a certain amount of time (100ms by default) has passed since the last time the browser window's size changed.

The debounce time can be changed via ```Holder.run({debounce:<time>});``` Make if falsey to disabled it completely.