Add a plugin architecture allowing registration of custom types

[stefcameron]

Custom validators are currently the only way to verify a value on an attribute that isn't natively supported by the library.

The library has been positioned in a way that should make supporting plugins possible, with its concepts of type validations and type validators, but there may be a lot of work remaining to do.

A plugin architecture would allow someone to register (i.e. add support for) a type and a validator (and perhaps a validation) such that it would then be treated as a known type when performing verifications.

Some thoughts are already in the code as comments, marked with TODO[plugins] headers.