v0.16.0

Added codeGen directive #93

Added a codeGen directive that is erased during source code generation and
can be used to customize the generated code.

Example - skip code generation

To skip the code generation and provide your own type.

query CodeGenHeroNameQuery {
  hero @codeGen(useType: "Hero") {
    name
  }
}

You can also use the fully qualified class name to avoid clashes

query CodeGenHeroNameQuery {
  hero @codeGen(useType: "com.example.model.Hero") {
    name
  }
}