Support directives: @skip, @include

[Totktonada]

It seems that grapql-lua supports directives already, so we need to write a test to check all works as expected. The example can be found here: http://graphql.org/learn/queries/#directives . I think the test can reuse the common_testdata dataset and conditionally include/skip a connection field. I think we need to check only a boolean variable as the expression of a directive and postpone more complex cases when it will be requested explicitly.