Expose tableview search string

So I found an instance where I needed to know what the user searched for when they tap a table cell so I figured this would be a good addition to the searchable DSL.

You can access `search_string` to get the string that the table is actually being filtered on and `original_search_string` to get what the user actually typed into the search box.

Example usage would be:

```ruby
  def open_screen(args={})
    args.merge!({user_search_string: search_string}) if searching?
    open DetailScreen.new(args)
  end
```

Then the `DetailScreen` would have access to the user's actual search text.

Let me know if you want this PR to go to another branch other than master.

:airplane: `coding_on_a_plane++` :airplane: