title: get the date for xarray rolling min or max
body: <p>Calling <code>foo.rolling(time=100).mean()</code> will give the mean for a rolling period of 100 days.
  I would like to find the minimum and maximum values within this 100 day period. Easy with <code>foo.rolling(time=100).min()</code></p>
  
  <p>However, how can I find the exact day of this minimum and maximum? I want to know what dates the min and max periods are during this rolling 100 day window. Or will I need to write my own function to do this?</p>
  
  <pre><code>num=4000
  data = np.random.rand(num, 3)
  locs = ['one', 'two', 'three']
  times = pd.date_range('2000-01-01', periods=num)
  foo = xr.DataArray(data, coords=[times, locs], dims=['time', 'space'])
  
  foo.rolling(time=100).max()
  foo.rolling(time=100).min()
  </code></pre>
  
  <p>Thanks </p>