dat documentation fixes

• dat export needs -d flag now, and doesn't give keys.

  dat export --help

  dat export --dataset= [--limit=] [--format=json,csv]

  Stream a range of rows out of a dataset. Dataset name required.

• dat delete is not documented here

  dat delete --help

  dat delete -d -m

  Delete the given key in the dataset.

• dat replicate is not in the dat command list but dat replicate --help still works...?