Add support for C# constants

[byme8]

You can create constant variables in C#. If you pass it in the GraphQL lambda it would correct C# syntax but the query will fail in runtime.

This PR adds support for constant by embedding their value into the GraphQL query itself. here example:

const string firstName = "John";
const string lastName = "Smith";
var response = await qlClient.Mutation(m => m.AddUser(firstName, lastName, o => o.Id));
// mutation { addUser(firstName: "John", lastName: "Smith") { id } }                    

Fixes #108