Support custom directives

It would be nice to add support of custom directives, since currently it is only available via reflection to get `_schema` from `GraphQLEngine` and invoke `RegisterDirective` passing `DirectiveGraphType`.
There are few ways:

- Add public getter to access `_schema`, so that any custom operation wich is possible in _graphql-dotnet_ will be possible with _graphql-dotnet conventions_
- this is the simplest but cons is that it only possible after `BuildSchema` executed
- Intoduce `WithDirective(DirectiveGraphType)` method which will be just a proxy to `_schema`
- it can remember provided type until `BuildSchema` is called or register directory immediately to `_schema` if it is already constructed
- Add adapter as for Queries and Mutations so that is would be possible to describe directive like a class:
```cs
[Name('mydircetive')]
[DirectiveLocations(DirectiveLocation.Field)]
class Directive {
public int Count { get; set; } // some argument
}
```