Fix #26: allow nested elements

Change the regexp in `isWS()` to only match space characters outside of element tags. The space in `<div class="foo">` is not matched, but the spaces in `<div>This is inner content</div>` are matched.

To make this more efficient, potential breakpoint indices could be held in an array outside of isWS.

A working demo can be seen [here](http://codepen.io/rileyjshaw/pen/oggBxz).