Add plugin return values for ImageViewer

The model supported is that plugins can return an overlay, some data,
or both. Each plugin therefore returns an `(overlay, data)` tuple in
which each element can be `None`.

To allow return values, the plugin need only override the `output`
method defined in the base Plugin class.

See discussions here:
https://groups.google.com/d/msg/scikit-image/0nkJM-WguXA/iqogBABa748J
and here:
https://github.com/scikit-image/scikit-image/pull/805

skimage/viewer/plugins/base.py

@@ -235,3 +235,20 @@ class Plugin(QtGui.QDialog):
         """Remove artists that are connected to the image viewer."""
         for a in self.artists:
             a.remove()
+
+    def output(self):
+        """Return the plugin's representation and data.
+        
+        Parameters
+        ----------
+        None
+        
+        Returns
+        -------
+        overlay : array, same shape as ``self.image_viewer.image``, or None
+            The filtered image.
+        data : array, arbitrary shape and type, or None
+            Any data associated with the plugin.
+        """
+        return (self.image_viewer.image, None)
+

skimage/viewer/viewers/core.py

@@ -228,6 +228,7 @@ class ImageViewer(QtGui.QMainWindow):
         self._show()
         if main_window:
             utils.start_qtapp()
+        return [p.output() for p in self.plugins]
 
     def redraw(self):
         self.canvas.draw_idle()