Support repeatable directives - progressive branch

[bbakerman]

I have created this support_repeatable_directives branch to incorporate the great work done in PR #1915

Since this work is not complete I wanted to create a long lived central branch that further work could be pushed to.

@dugenkui03 can you base any future work off this branch. We will merge master on to it periodically.

This will allow us to work collaboratively.

The code as is does query repeatable directives

However we need to revisit the SchemaGenerator and GraphqlXXX types so that directives are truly everywhere including SDL types, We also need to put in support inside GraphqlDirective itself for the repeatable boolean.

References :

#1763

https://github.com/graphql/graphql-spec/pull/472/files

[dugenkui03]

Thanks for your suggestions and review. It will be great with your support.
More work in #1986 .

[bbakerman]

See #2015