Add support for <r:edit_mode_only />

[adamjohnson]

Docs:

http://cleanslatecms.wvu.edu/how-to/theme-development/tag-index/r-edit-mode-only

This tag can be used to cheaply do something like page/image attributes. Aka you don't want to have it visible on page but maybe you want to pull this {thing} into it's parent or onto another page.

<r:edit_mode_only>
    <p>I only want to see this when editing the page.</p>
</r:edit_mode_only>

I think the contents of the tag should be output onto the page in Hammer. Not sure if you want to add a message like: "Hey, this will only appear in edit mode!" or something like that.