mu4e-view: select next unread message in view buffer

Previously, the behavior of mu4e-view-headers-next-unread was that the cursor moved in the headers view but did not change the highlight nor the message in the view buffer. This patch updates the function to also select the message in the view buffer which appears to also highlight the message correctly in the header view.
Merge pull request #530 from seanfarley/master

mu4e-view: select next unread message in view buffer