generate header id the same as github

[livelazily]

for example:

Original marked:

var marked = require('marked');

var input = '### CollectionView\'s `getItemView`';

console.log(marked.parse(input));
//=> <h3 id="collectionview-s-getitemview-">CollectionView&#39;s <code>getItemView</code></h3>

the h3's id is collectionview-s-getitemview-

but in github, the id is collectionviews-getitemview
github just ignore all non word characters

[SamyPesse]

Sorry for taking so long, can you rebase on master so that we can merge it.