title: Grunt required options for specific task
body: <p>I'd like to fail a grunt task before executing if a certain cli option is not present. At present I have the following:</p>
  
  <pre><code>module.exports = function(grunt) {
    grunt.initConfig({
      mytask: {
        default: {
          launch_url: (grunt.option("url") || grunt.fail.warn("Set url with --url.")),
        },
        options: {}...
  </code></pre>
  
  <p>There must be a cleaner way to do this- i just want the cli to require a url at runtime rather than raising some unintelligble error later.</p>