add validation of directives in SDL

From #1383:

It’s very critical to add validation of directives in SDL since as noted here no one validates them correctly at the moment. That will lead to a situation very similar to the recent Allow interfaces to have no implementors. Since spec defines set of validation rules but no one implements them. Probably we shouldn’t enforce Directives Are Unique Per Location until https://github.com/facebook/graphql/issues/429 resolved but all other validation should be implemented e.g. require the schema to contain applied directives, correct directive arguments, etc.

[yaacovCR]

Well, validated in a sense that the directives have to exist, I don’t know if the arguments are validated / coerced yet…

[IvanGoncharov]

@mjmahone Update: I finally figured out how to solve the last major problem. I will open a couple of PRs implementing this feature in the next few hours.