title: how do I calculate a rolling idxmax
body: <p>consider the <code>pd.Series</code> <code>s</code></p>
  
  <pre><code>import pandas as pd
  import numpy as np
  
  np.random.seed([3,1415])
  s = pd.Series(np.random.randint(0, 10, 10), list('abcdefghij'))
  s
  
  a    0
  b    2
  c    7
  d    3
  e    8
  f    7
  g    0
  h    6
  i    8
  j    6
  dtype: int64
  </code></pre>
  
  <p>I want to get the index for the max value for the rolling window of 3</p>
  
  <pre><code>s.rolling(3).max()
  
  a    NaN
  b    NaN
  c    7.0
  d    7.0
  e    8.0
  f    8.0
  g    8.0
  h    7.0
  i    8.0
  j    8.0
  dtype: float64
  </code></pre>
  
  <p>What I want is</p>
  
  <pre><code>a    None
  b    None
  c       c
  d       c
  e       e
  f       e
  g       e
  h       f
  i       i
  j       i
  dtype: object
  </code></pre>
  
  <p>What I've done</p>
  
  <pre><code>s.rolling(3).apply(np.argmax)
  
  a    NaN
  b    NaN
  c    2.0
  d    1.0
  e    2.0
  f    1.0
  g    0.0
  h    0.0
  i    2.0
  j    1.0
  dtype: float64
  </code></pre>
  
  <p>which is obviously not what I want</p>