[imap] configurable content-type selection for multipart messages

User can now select which content-type in multipart messages should be
used as the event message.  Default behavior for multipart messages is
still to select first part with text/plain content-type.
Merge pull request #784 from bfritz/imap-config-content-type

[imap] configurable content-type selection for multipart messages

Looks good (Jordan approves)!  Thanks for making Logstash awesome! :)