some comments on Stix-2.1

To recap some of my thoughts regarding Stix-2.1:

"description" (optional) should be part of the common properties.
"name" (required) should be part of the common properties.
"labels" which is part of the common properties should either be required or optional, not both.

With respect to Observable EmailMessage, I think there is no need to have "is_multipart" and "body_multipart”. Make "body" a dictionary of "mime-part-type" (string) and "body_part" (string).