title: "Insert before" for std::list
body: <p>Lets say I have a linked list which contains the following:</p>
  
  <p>1, 2, 4, 5, 6, 2, 3, 2</p>
  
  <p>now lets say I wish to convert this to:</p>
  
  <p>1, 10, 2, 4, 5, 6, 10, 2, 3, 10, 2</p>
  
  <p>i.e. insert 10 before all 2's. </p>
  
  <p>How should I do this? </p>