Allow select fields of flexinodes appear in the search interface

There should be a way to display select fields of a flexinode to appear as limiting criteria of an "advanced" search interface.

It would display the checked fields to display below the search textfield and allow the user to choose which fields to limit their searches.

It should be handled in the field level and each field type would be responsible to providing the proper means of searching through the content.

I'm still unclear of how the search hooks work for flexinode, so I'll write this for now.