Richtext component restproperty

[hypervillain]

Following #35, we needed a way to pass props (like className) to the wrapper element created by RichText.

Given this call to RichText:

<RichText
 render={myField}
 Component="div"
 className="my-class-name"
 data-attribute="data"
/>

Your HTML markup should end up like this:

<div class="my-class-name" data-attribute="data">
    /* ... */
</div>

Note: users might forget that wrapper Component defaults to React.Fragment and therefore will not pass rest properly.