title: WooCommerce: Add-on option label doesn't take negative price
body: <p>I'm using WooCommerce v. 2.1.7. I'm also using the following skinning plugin, which made my skinning customizations easier:
  <a href="http://themeforest.net/item/mommerce-modern-multipurpose-woocommerce-theme/6727886" rel="nofollow noreferrer">http://themeforest.net/item/mommerce-modern-multipurpose-woocommerce-theme/6727886</a></p>
  
  <p>I have a product page that looks something like this:</p>
  
  <p><img src="https://i.stack.imgur.com/5Aw6z.png" alt="enter image description here"></p>
  
  <p>I'm trying to add a price to "Try on" a dress for less than the 7-day rental price ($910). It's set up like the following in the WooCommerce backend:</p>
  
  <p><img src="https://i.stack.imgur.com/tMUy6.png" alt="enter image description here"></p>
  
  <p>When I try to select "New Option" and add a -875 or any negative number, WooCommerce screams at me. How can I get this to accept a negative value? </p>
  
  <p>Ideas I thought of:</p>
  
  <ol>
  <li>Where do I find, and remove, the validator that makes WooCommerce scream at me for entering a negative value?</li>
  <li>Where exactly do I find the code in this system to hard code the try-on price value?</li>
  </ol>