Non standard properties in dialogs throw errors.

[howethomas]

Inbound vCons have non-standard elements like "id" in the parties. Example attached. In order to make it work, we need to do this:
# Move the id from the parties to metadata if it's there
for party in vcon_json["parties"]:
if "id" in party:
party["meta"] = party.get("meta", {})
party["meta"]["id"] = party["id"]
del party["id"]

Improve the library by giving options on how to handle this situation for all embedded objects (Party, Dialog, Analysis and Attachment):

• By default, add these non-standard properties to the new vCon, do not throw an error
• Add a property to the constructor to control the alternatives:
• strict, which removes all non-standard properties
• meta, which takes all non-standard properties and puts them into the meta object (as above)