STY: Clean up formatting and remove repeated imports

doc/source/user_guide/viewer.txt

@@ -8,29 +8,31 @@ Quick Start
 
 ``skimage.viewer`` provides a matplotlib_-based canvas for displaying images and
 a Qt-based GUI-toolkit, with the goal of making it easy to create interactive
-image editors. You can simply use it to display an image::
+image editors. You can simply use it to display an image:
 
-   >>> from skimage import data
-   >>> from skimage.viewer import ImageViewer
+.. code-block:: python
 
-   >>> image = data.camera()
-   >>> view = ImageViewer(image)
-   >>> view.show()
+   from skimage import data
+   from skimage.viewer import ImageViewer
+
+   image = data.coins()
+   viewer = ImageViewer(image)
+   viewer.show()
 
 Of course, you could just as easily use ``imshow`` from matplotlib_ (or
 alternatively, ``skimage.io.imshow`` which adds support for multiple
 io-plugins) to display images. The advantage of ``ImageViewer`` is that you can
 easily add plugins for manipulating images. Currently, only a few plugins are
 implemented, but it is easy to write your own. Before going into the details,
-let's see an example of how a plugin is added to the viewer::
+let's see an example of how a plugin is added to the viewer:
 
-   >>> import skimage
-   >>> from skimage.viewer.plugins import Canny
+.. code-block:: python
 
-   >>> image = skimage.img_as_float(data.camera())
-   >>> viewer = ImageViewer(image)
-   >>> viewer += Canny(view)
-   >>> viewer.show()
+   from skimage.viewer.plugins import Canny
+
+   viewer = ImageViewer(image)
+   viewer += Canny(view)
+   viewer.show()
 
 At the moment, there aren't very many plugins pre-defined, but there's a really
 simple interface for creating your own plugin. First, let's create a plugin to
@@ -70,10 +72,6 @@ All that's left is to create an image viewer and add the plugin to that viewer.
 
 .. code-block:: python
 
-   from skimage import data
-   from skimage.viewer import ImageViewer
-
-   image = data.coins()
    viewer = ImageViewer(image)
    viewer += denoise_plugin
    viewer.show()