Update Shortcode.js

update replaceNodes to query this.el and not document.

reason:
```
var el = document.createElement('span');
el.innerHTML = '[hello]Good Morning World[/hello]';

new Shortcode(el, {
  hello: function() {
    return this.content;
  }
});
```

The above will not work if replaceNodes function queries document when parsing.
Merge pull request #5 from dex-labs/document-to-element

Update Shortcode.js