v0.2.0

Added a new API Resolver.Scan:

Scan scans the struct value by traversing the fields in depth-first order. The value is required
to have the same type as the resolver holds. While scanning, it will run the directives on each
field. The DirectiveRuntime that can be accessed during the directive exeuction will have its
Value property populated with reflect.Value of the field. Typically, Scan is used to do some
readonly operations against the struct value, e.g. validate the struct value, build something
based on the struct value, etc.