Dataframe extension integration

+ The unfortunate thing about using `docopt` defaults is that they are static unless you add some magic. Since `--alias` does have a default, but which isn't static (it depends on your configuration), it can't be defined here. For consistency/simplicity, I like the idea of not including any `docopt` defaults. This is a equally valid choice as well though.
+ A few typos (`downloadeding`, `Ovewrite`).