Add "Diffux" to the user-agent used when snapshotting

This helps if you want to single out Diffux as a bot when you render a
page. I'm only adding it in cases where we would otherwise use the
default PhantomJS one.
Use box-shadow to highlight current diff cluster

Before, we were using a 1px gray border to the top and bottom. By
changing that to two box-shadows (one dark and one light), we get the
following benefits:

- The border is visible on darker pages too
- The border is rendered outside the cluster (before we were cutting off
  2px)