Filter directives

[Václav Pelíšek]

New executive directive which filters list items.

query { fieldListOfSomething @filterField(name: "foo", contains: "bar") {
   foo 
}}

outputs only items where foos value contains "bar". Support for subfields:

query { fieldListOfSomething @filterField(name: "foo.bar.baz", contains: "bar") {
   foo { bar { baz }}
}}

[Václav Pelíšek]

Continues in #98