enable tab completion

When you're cranking, its annoying to have to type relatively long `creates` or `alias` names. It would be really nice if you could just tab complete on `creates` targets so that something like this would happen

```bash
[terminal]$ workflow da[TAB]
[terminal]$ workflow data/[TAB]
reuters21578.tar.gz
reuters21578
corpus.csv
standardized_corpus.csv
tfidf.csv
```

I haven't dont any research into how feasible this is, but it seems like argparse is considerably more awesome and could conceivably make this relatively easy.

https://github.com/kislyuk/argcomplete