mu4e: Make :show-target be the identity by default

- This is certainly a better default than crashing
- The user is likely to omit this function in a majority of cases.
take a stab at the documentation of mu4e-marks
Merge pull request #527 from jyp/master

mu4e: Document the mu4e-marks variable in the manual
minor doc fixes
mu4e: allow lisp expressions for bookmarks

based on an idea by davidcsterratt
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
Add a key binding for message-view doc.
mu4e: add another example for using marks to tag a message

This example shows how to archive and mark a message as read which emphasizes
the importance of order in tagging a message before anything else due to the
implementation of tagging (since it currently uses 'sed').
Merge pull request #532 from seanfarley/master

mu4e: add another example for using marks to tag a message
Merge pull request #531 from xuhdev/message-view-doc

Add a key binding for message-view doc.
mu4e: append `substring' to the completion-styles for address-completion