title: moment().add() only works with literal values
body: <p>I'm using Moment.js in TypeScript (under Angular 2 if that matters).  When I use the add() method with literal values as arguments, it works fine:</p>
  
  <pre><code>moment().add(1, 'month');
  </code></pre>
  
  <p>However, if I try to replace the units with a string, it fails:</p>
  
  <pre><code>let units:string = 'month';
  moment().add(1, units);
  </code></pre>
  
  <p>with this error:</p>
  
  <pre><code>Argument of type '1' is not assignable to parameter of type 'DurationConstructor'.
  </code></pre>
  
  <p>What am I doing wrong here?</p>