support external types

[mafintosh]

ie

ns.register({
  name: 'my-custom-type',
  external: './file/that/has/getStruct/export.js'
})

When the code is required, it would call getStruct(fqn) on the above file expecting to get the requested struct back. Doing it this also allows you to chain generated schemas in addition to allowing "hand rolled" encoders