Support for multiple keys

[jonnii]

Apollo Federation supports multiple primary keys. We could implement this in graphene-federation in one of two ways:

# Multiple key decorators
@key("id")
@key("username")
class Person(ObjectType):
    pass

# multiple keys in a single decorator
@key("id", "username")
class Person(ObjectType):
    pass

Can you let me know if you have a preference and I'll send a PR.

[jonnii]

@erebus1 I opened #12 just in case.

[erebus1]

merged