Extend redshift_sqlalchemy by UNLOAD and COPY support

/review @chase-seibert @newtonry 
I am not sure what you mean. The `options` dict is nested inside `element`. The individual contents of the `element.options` are explained in detail in the comments of lines 187-198, which is the description of the __init__ function of the UnloadFromSelect class that should be used by external callers to parameterize the UnloadCommand