title: WooCommerce - Add another scheduled sale price
body: <p>I will try to explain what I need:</p>
  
  <p>By default woocommerce allows to handle just 1 scheduled sale price per product, I need the possibility to add another one (for simple and variable products).</p>
  
  <p>In other words, for each product I need to have 3 different prices, depending on the date the purchase is made. 
  Also, in the product page I need to show ALWAYS the 3 prices and their relative scheduled period of time (even if it's in the past or future). 
  Example: </p>
  
  <p>Product #1 </p>
  
  <ul>
  <li>from 01-01-2014 to 01-31-2014 you pay € 100 (this is what I need)</li>
  <li>from 02-01-2014 to 02-28-2014 you pay € 200 (scheduled woocommerce</li>
  <li>sale price) from 03-01-2014 and after you pay € 300 (regular woocommerce price)</li>
  </ul>
  
  <p>Product #2 </p>
  
  <ul>
  <li>from 01-01-2014 to 01-31-2014 you pay € 150 (this is what I need)</li>
  <li>from 02-01-2014 to 02-28-2014 you pay € 280 (scheduled woocommerce sale price)</li>
  <li>from 03-01-2014 and after you pay € 330 (regular woocommerce price)</li>
  </ul>
  
  <p>etc...</p>
  
  <p>Any suggestion on how to add this feature in a new plugin?</p>