0.9.0

[sikanhe]

• No longer need create a builder just to have user provide GqlContext type
• No need for DefaultArg vs Arg, (leverage conditional types)
• Union/Interface resolveType now returns string, as expected by graphql-js v16
• unionType's types array can be defined as a function to support forward references
• All type constructors are exported under "Gql" namespace for easy import.