Support auto-completion of Metaflow objects/API in notebooks

We could support some form of auto-completion for Metaflow in iPython. Some examples:
```
def _ipython_key_completions_(self):
    return [s.id for s in self.steps()]
```
on a run to complete something like `my_run['<tab>`

Details on the possibilities here: https://ipython.readthedocs.io/en/stable/config/integrating.html#tab-completion