Support class level decorators

[rrousselGit]

A potential solution for #49

The principle is, any decorator added on a @freezed constructor would be transposed to the generated class.

This means that if we write:

@freezed
abstract class Example with _$Example {
  @deprecated
  factory Example() = GeneratedClass;
}

This would lead to:

@deprecated
class GeneratedClass {
...
}