[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.

Use case:
Third party system is sending non-multipart email with HTML body to mailman list.  mailman converts the email to multipart with `text/html` part for original body plus `plain/text` part for the list footer.

Would appreciate a second set of eyes on the tests.  I rarely use ruby and this my first dance with rspec.