Implement command to list models (local and downloadable)

[pmarini-nc]

As mentioned in #26, typing occ translate: triggers a download of all the models (that is, it is effectively an alias of occ translate:download-models).

I believe this is unexpected from the user perspective, as in general occ <app-name>: is used to obtain the list of commands exposed by the app.

I'd suggest to create a command to list models, both those available locally (that is, that have already been downloaded) and those that can be downloaded.

Implementing this command, beyond being useful, would avoid the behaviour outlined above.