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
Curious: I guess you did `replace(/\s+/g, " ")` above for a reason instead of `text.split(/\s+/)`, right? What is the catch with that?