Add a brushend event.

When dealing with many observations and when trying to connect pc to other kinds
of views, it is more desirable to update views only on brushend. Updating on
each brush event causes a lot of unnecessary rendering in other linked views.
Add a function to retrieve current brush extents.

I need this for an application I'm working on. However, this can also serve as a
starting point for fixing the brush resets on resize. I'll have a look at that
soon.