Reader/Writer decorators to simplify definitions

Allow for methods that accept `reader` as their argument to have a `@provides_reader` decorator to pass in a valid reader from a Data String, file, or IO object.

Allow for methods that accept `writer` as an argument to have a `@provides_writer` decorator to generate a writer if one is not passed. This should also throw an error if a writer is not returned from a `save()` function.
