Improve support of sub/superscript

![sub-super-strikethrough](https://cloud.githubusercontent.com/assets/1052079/3809642/8064f344-1c86-11e4-9597-eea9439d0b3d.png)


Test document: https://drive.google.com/file/d/0B1EH5OPb-RrjbW9DQ1VRLUxQUEE/edit?usp=sharing
Contrary to my initial expectations, ``text.split(/\s+/)`` creates a bunch of empty strings in the array as well, rather than consuming multiple spaces as one separate :-/