Add support for directives in schema parser

[Ackar]

This adds support for parsing directives in the schema.

It's probably of limited use for now but it's mostly so it stops crashing on schemas with directives, and it's a first step towards a better directive support.

[tsholmes]

This would enable us to fully autogenerate the go interfaces to match the schema using directives for some configuration options. @tonyghita what needs to be done to get this merged?

[tsholmes]

@pavelnikolov @tonyghita ping

[pavelnikolov]

Thank you very much for your contribution.