generate header id the same as github

for example:

Original marked:
```js
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