Add stub command

[vektah]

WIP - very incomplete

This adds a new gqlgen stubs command that compares the generated interfaces and creates stubs for any missing ones, updating and signatures along the way.

Because this is mutating existing user-written source code I want to keep it in a separate command for now. As we gain some confidence in it I imagine it would happen automatically in gqlgen generate if configured.

Fixes #216

I have:

• Added tests covering the bug / feature (see testing)
• Updated any relevant documentation (see docs)

[mathewbyrne]

Closing for now, some of this might be useful later.

[frederikhors]

@mathewbyrne why later and not now?

[mathewbyrne]

why later and not now?

I'm not sure what you mean. This PR itself will never merge, but some sections of it may be useful when we do address stub generation at some point. We are currently addressing some more fundamental refactors to code generation and type mapping that are more of a priority at the moment.