Make ExtractResults list/dict friendly

`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
```
