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
So could you please add a note about that (and mention where it fails), to prevent somebody "improving" this? E.g. on my Firefox and Chromium it works as expected:
```
"   a   b  ".replace(/^\s*(.*?)\s*$/g, "$1").split(/\s+/)
["a", "b"]
```