add paragraphFn option to control the type of React.DOM component output

Love the work you've done here, but I'd like to get paragraph tags out of marked-react for paragraph blocks.

This adds an option to preserve the existing behavior:

```js
marked.setOptions({
  paragraphFn: function(text) {
    return React.DOM.div({className: 'paragraph'}, text);
  }
});
```

Example use:
```
rcs at Boreas in ~/Devel/marked-react on master
❯ node
> global.React = require('react');1;
1
> marked = require('.');1;
1
> React.renderComponentToString(marked('__example text__')[0])
'<p data-reactid=".smjfreqzgg" data-react-checksum="-309317655"><strong data-reactid=".smjfreqzgg.0"><span data-reactid=".smjfreqzgg.0.0">example text</span></strong></p>'
> marked.setOptions({paragraphFn: function(text) { return React.DOM.div({className: 'paragraph'}, text); }});1;
1
> React.renderComponentToString(marked('__example text__')[0])
'<div class="paragraph" data-reactid=".iyr85qij9c" data-react-checksum="-295487969"><strong data-reactid=".iyr85qij9c.0"><span data-reactid=".iyr85qij9c.0.0">example text</span></strong></div>'
>
```