Fix custom function usage with assignable types

Now conversion and inspection functions are skipped for fields of directly assignable or convertible types, so this operation can be performed if there is no other way to do the mapping

Please open an issue if it breaks your use case

Allow ignoring fields with struct tags

Thanks to @Osendo for this feature! 🔥

type Order struct {
	Id string

	json.Marshaler   `dto:"ignore"` // ignored
	json.Unmarshaler `dto:"ignore"` // and this as well
}

Small pointer update

dto-mapper now supports mapping pointers of arbitrary types and initializing nil values upfront. See the TestPointerCases testcase for examples.

This closes #1