title: How to increase/decrease current margin at a number by jquery?
body: <p>Given an element with unknown margin-left, how to increase its margin-left at a number say 100px?</p>
  
  <p>For example:</p>
  
  <p>assuming the original margin-left is 100px</p>
  
  <p>the expected result is 100px + 100px thus 200px finally.</p>