title: Reverse-Mapping for String Enums
body: <p>I wanted to use string enums in typescript but I can't see a support for reversed mapping in it.
  I have an enum like this:</p>
  
  <pre><code>enum Mode {
      Silent = "Silent",
      Normal = "Normal",
      Deleted = "Deleted"
  }
  </code></pre>
  
  <p>and I need to use it like this:</p>
  
  <pre><code>let modeStr: string;
  let mode: Mode = Mode[modeStr];
  </code></pre>
  
  <p>and yes I don't know what is it there in <code>modeStr</code> string and I need it parsed to the enum or a fail at parsing in runtime if the string is not presented in the enum definition.
  How can I do that as neat as it can be?
  thanks in advance</p>