New flexible method of customising completing-read

Added the customisation option 'mu4e-completing-read-function' which can
be used to choose which method of input-with-completion will be used by
mu4e. This variable is set to the function which mu4e will use.

By default, mu4e will use ido (via 'ido-completing-read'), but any
compatible completing-read function can be used instead. Interesting
choices would include 'helm-comp-read' for Helm-based completion, and
'completing-read' for the built-in basic completion system.
mu4e: always consider text/calendar message parts as attachments
mu4e: add long datetime tooltip to human-date field in headers view.
Merge pull request #268 from hickinbottoms/completing-read-choice

* new flexible method of customising completing-read
* update doc for mu4e-completing-read-function
Merge pull request #271 from abend/icalendar-as-attachment

* mu4e: always consider text/calendar message parts as attachments
Merge pull request #272 from abend/long-date-tooltip

* mu4e: add long datetime tooltip to human-date field in headers view.
* mu4e: make the mu4e-headers-long-date-format default a bit more... default