Custom Directives Support

[pavelnikolov]

I've been working on custom directives in this library for a while. I got @auth(role: String!) working just fine. I also got query complexity directive e.g. @cost(wieght: 1, multiplier: "pageSize) working - but I'm polishing the API and docs before merging this code. Here is my progress so far - tests passing (see executable examples for tl;dr) #760 - this handles pre-execution directives on FIELD_DEFINITION.
I'd appreciate a review if you have the time. I still haven't thought much about reusable custom field validation directives e.g. @email, @zipCode, @gt(n: Int!), @regexp(match: String!), etc. Also, at this point, no support for changing the output or skipping exec e.g. @cache(key: "ID", ttl: "1h").

I need feedback on the API, not on the implementation.