move message-modifying methods from Imbox to Struct

[zevaverbach]

To be more "for humans". 😄 Would you accept a PR implementing this?

[zevaverbach]

Also, having done this, update a Struct's fields whenever such a method is called.

[zevaverbach]

@martinrusev having made this change, Struct (aka Message) could have lots of properties for flagged, read, unread, etc.

[zevaverbach]

>>> messages = Imbox.messages()
>>> m = messages[0]
>>> m

Message(sent_from='[email protected]'
        date_sent=datetime.datetime(2018, 7, 27, 14, 37)
        subject='thank you for merging my PR'
        body='')

>>> m.read
True
>>> m.unread
False
>>> m.mark_as_unread()
>>> m.unread
True

[zevaverbach]

@martinrusev checking in with you on this. Would you accept a PR implementing this?

[martinrusev]

@zevaverbach that will be a good addition.

[erSitzt]

was there a PR ?`