Make ExtractResults list/dict friendly

[msukmanowsky]

ExtractResult instances should implement Iterator and getitem in the event users prefer to work with them as shown:

er = ExtractResult(...)
parts = list(er)
er['engine_name'] == er.engine_name

Maybe even nice to use namedtuples here to just auto-unpack values to something like:

ename, kw, parser = er