title: Add plus sign "+" before input value in Angular2
body: <p>The question sounds pretty easy, but I could found any clear solution for it.</p>
  
  <p>I have custom input field (I'm using reactive forms approach) for phone numbers. The aim is to always have a plus sign before input value.</p>
  
  <p>Requirements:</p>
  
  <ul>
  <li>the entire input value including plus sign should be selectable (i.e. the hack with adding span-s or div-s with plus sign before input is not acceptable)</li>
  <li>if the input value is empty - the plus sign can be removed, otherwise it should be added at the beginning of the input value if we add numbers</li>
  <li>using third party libraries is not acceptable </li>
  <li>when patching the form control with the new value (for example '123456') - it should also add plus sign before</li>
  </ul>
  
  <p>How can we implement this logic in Angular2 project?</p>