Add some snippet

add 'extend' snippet

add 'include' snippet

add the possibility to avoid the type inside "link css" not necessary in HTML5

add role 'snippet'

add "data-" snippet

add 'form' snippet

add 'aria-*' snippet

add 'itemprop' snippet

add 'itemscope' snippet

add 'itemtype' snippet
updated documentation
Merge pull request #1 from rs459/master

Some addition to provide a better autocompletion