title: Create subtasks that override `options` but use task level `src`
body: <p>My gruntfile thus far</p>
  
  <pre><code>uglify: {
      src: [
          ...
      ],
      dest: 'js/application.min.js',
      options: {
          'compress': {},
          'reserveDOMCache': true,
          'enclose': undefined,
          'exportAll': false,
          'expression': false,
          'preserveComments': false,
          'report': 'min',
          'sourceMap': false,
          'sourceMapIn': undefined,
          'sourceMapIncludeSources': false,
          'sourceMapName': undefined,
          'wrap': undefined
      },
      development: {
          options: {
              'beautify': false,
              'mangle': true
          }
      },
      production: {
          options: {
              'beautify': true,
              'mangle': false
          }
      }
  }
  </code></pre>
  
  <p>However when I run the task <code>uglify:development</code> it will respond with <code>No files created.</code></p>