title: Merging two dataframes based on a date between two other dates without a common column
body: <p>I have two dataframes that I need to merge based on whether or not a date value fits in between two other dates. Basically, I need to perform an outer join where <code>B.event_date</code> is between <code>A.start_date</code> and <code>A.end_date</code>. It seems that merge and join always assume a common column which in this case, I do not have. </p>
  
  <pre><code>    A                           B
      start_date  end_date        event_date  price
  0   2017-03-27  2017-04-20  0   2017-01-20  100
  1   2017-01-10  2017-02-01  1   2017-01-27  200
  
  Result 
      start_date  end_date        event_date  price
  0   2017-03-27  2017-04-20  
  1   2017-01-10  2017-02-01      2017-01-20  100
  2   2017-01-10  2017-02-01      2017-01-27  200
  </code></pre>