Add HTML5 display definition

- Correct `block` display not defined for any HTML5 element in IE 8/9.
- Correct `block` display not defined for `main` in IE 11

This has repeatedly tripped people up (myself included), so it might be
time to add it to the template.
Merge pull request #97 from gemmaleigh/add-display-block-to-main

Add HTML5 display definition