feature: Prefix type names

For example, given the schema:

interface A {
  id: ID!
}

type B implements A {
  id: ID!
  b: String!
}

With a prefix of GQL_, would generate:

• interface GQL_A
• class GQL_B implements GQL_A

This would help when converting between database, RPC, and GraphQL objects sharing the same name for example

I know #21 and #132 exist, would a PR in the (distant) future be OK?