Add metadata info

add an option to display metadata for a variable in a dataset

via command line:

$ psyplot tests/test-t2m-u-v.nc -i t2m

returns

t2m:
  Attributes:
    code: '130'
    grid_type: gaussian
    long_name: Temperature
    table: '128'
    units: K
  Time information:
    Coordinate: time
    Dimension name: time
    Shape: ('time',) -> (5,)
  Vertical information:
    Coordinate: lev
    Dimension name: lev
    Shape: ('lev',) -> (4,)
  X-Coordinate information:
    Coordinate: lon
    Dimension name: lon
    Shape: ('lon',) -> (192,)
  Y-Coordinate information:
    Coordinate: lat
    Dimension name: lat
    Shape: ('lat',) -> (96,)

---

Update

in !65 (merged), this option has been changed such that -i does not take arguments. the above command becomes

$ psyplot tests/test-t2m-u-v.nc -i -n t2m