title: convert from year-month-date to year-Quarter in pandas
body: <p>I have a pandas dataframe, where one column(first_date) contains date in the following format:
  <code>2018-03-31</code> </p>
  
  <pre><code>#I have created a monthly level data using the below code
      dataset['first_date']=pd.to_datetime(dataset['first_date']) 
      dataset['first_date'].apply(lambda x: x.strftime('%Y-%m'))
      **output: 2018-03**
  </code></pre>
  
  <p><strong>My Question</strong>:​ using first_date I want to create a new column, to get the year &amp; quarter.
  output something like <strong>2018-Q1</strong> and must be in string not in <strong>periods</strong></p>