Merge improvements from https://github.com/dgraph-io/gqlparser

[StevenACoffman]

https://github.com/dgraph-io/gqlparser has been maintaining their own fork of the parser, and they have made a number of improvements. I would like to upstream these. I have pushed their current commits to a branch https://github.com/vektah/gqlparser/compare/dgraph-master?expand=1 in case their current difficulties cause their repository to become unavailable for any reason.

[geoff-nunan]

I'm in the same position as @StevenACoffman, and now that dgraph has officially been absorbed into another project, I'd like to move the dependencies in my dgraph fork back to gqlgen rather than being dependent on dgraph