allow typname to be a list

[chadwhitacre]

I have a model that I'd like to register for multiple types. The usecase is making a type that basically inherits from another type (though afaict Postgres doesn't implement type inheritance yet), overriding a foreign key field to actually subselect the record from the other table. By registering a single composite for multiple such types I could determine at query time whether I'd end up with the subselect or not.

[chadwhitacre]

Here's where this arose: gratipay/gratipay.com@1ec5522