title: Grunt uglify throws parse error when string started with ` character
body: <p>I am running grunt uglify task to minify my js files but it is giving me error when string started with ` character.</p>
  
  <p>My string look like this</p>
  
  <pre><code>var html = `&lt;div class='list-item'&gt;
                &lt;span&gt;ABC&lt;/span&gt;
             &lt;/div&gt;`;
  </code></pre>
  
  <p>My Gruntfile.js uglifyjs block is like this</p>
  
  <pre><code>uglify: {
    'dev': {
         options: {
            mangle: false
         },
         files: {
            'common/utility_functions.js': ['common/utility_functions.js']
         }
     }
  }
  </code></pre>
  
  <p>It is giving me following error when i run grunt uglify</p>
  
  <pre><code>&gt;&gt; Uglifying source common/utility_functions.js failed.
     Warning: Uglification failed.
     SyntaxError: Unexpected character '`'.
     Line 162 in common/utility_functions.js
     Use --force to continue.
  </code></pre>
  
  <p>Can anyone help to fix this issue? I think using ` character is valid. It has not given me error during jshint process, because i added enext: true in jshint options.</p>