Add support for custom selectors

[andrewcroome]

It would be nice to be able to add some special UI to support radio buttons for scopes. For example, in the following search input, we'd like users to be able to scope:

[x] All assets [ ] Uploaded by me to see either all assets, or only ones they've uploaded. If that control appended an uploaded_by:me string to the query string, we can look for that on the server side.