title: pandas.ExcelWriter ValueError: Append mode is not supported with xlsxwriter
body: <p>I want to add some records to an excel file and I use <code>pandas.ExcelWriter</code> to do this(<a href="http://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.ExcelWriter.html?highlight=excelwriter#pandas.ExcelWriter" rel="noreferrer">http://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.ExcelWriter.html?highlight=excelwriter#pandas.ExcelWriter</a>
  ):</p>
  
  <pre><code>import pandas as pd                                                     
  
  df = pd.DataFrame(data={'a':[4], 'b':['corn'], 'c':[0.5]})              
  
  with pd.ExcelWriter("test.xlsx", mode='a') as writer: 
              df.to_excel(writer) 
  
  </code></pre>
  
  <p>a, b, c are titles name of test.xlsx</p>
  
  <p>run this program, raise a valueError:</p>
  
  <pre><code>ValueError                                Traceback (most recent call last)
  &lt;ipython-input-3-c643d22b4217&gt; in &lt;module&gt;
  ----&gt; 1 with pd.ExcelWriter("test.xlsx", mode='a') as writer:
        2     df.to_excel(writer)
        3 
  
  ~/anaconda/lib/python3.6/site-packages/pandas/io/excel.py in __init__(self, path, engine, date_format, datetime_format, mode, **engine_kwargs)
     1935 
     1936         if mode == 'a':
  -&gt; 1937             raise ValueError('Append mode is not supported with xlsxwriter!')
     1938 
     1939         super(_XlsxWriter, self).__init__(path, engine=engine,
  
  ValueError: Append mode is not supported with xlsxwriter!
  </code></pre>
  
  <p>I don't know why?</p>