Add 'main' html element

Add the missing 'main' html 5 element.
Before:
![2014_12_20_18_57_21_style less_d_prog_github_laedit net_atom](https://cloud.githubusercontent.com/assets/871092/5515780/9cd6c538-887a-11e4-8a36-7cae8769cd58.png)

After:
![2014_12_20_18_59_54_style less_d_prog_github_language_less_atom](https://cloud.githubusercontent.com/assets/871092/5515781/a3925ea0-887a-11e4-8f4a-11b57345e259.png)