Add describe command for CLI

[aguschin]

Command should give model description, basically, presenting what was saved in metadata. Something like this:

$ mlem describe dog-bark-translator --repo https://github.com/iterative/model-registry --rev main
 - 📖 Translates dog barks in emoji.
 - 📦 Pytorch 1.10.0, Torchaudio 0.10.0, Emoji 1.6.1
 - 🎯 Accuracy 87.3%

We don't track metrics yet, though showing them with this command would be definitely helpful.

[mike0sv]

Related to #43