describe improvement. Make type and path optional for annotation.

[aguschin]

Now describe works like this:

$ gto --tb describe rf
{'name': 'rf', 'type': 'model', 'path': 'models/random-forest.pkl', 'virtual': False, 'tags': [], 'description': ''}

$ gto --tb describe rf --path
models/random-forest.pkl

address #122