title: how do I get at the pandas.offsets object given an offset string
body: <p>Suppose I have an offset string <code>'BM'</code> or <code>'7W'</code><br>
  I know the answer for <code>'BM'</code> is <code>pd.offsets.BMonthEnd()</code><br>
  for <code>'7W'</code> is <code>pd.offsets.Week(7)</code></p>
  
  <p>Is there a generic solution in which I can pass a string and get the offset object?</p>