add minify options

Hi, I thought it should be possible to pass more options to the `uglify.minify` options so I made this little change that should work.

I wanted to be able to do something like this:

```js
module.exports = _grunt;
function _grunt (grunt) {
  grunt.initConfig({
    browserify: {
      dev: {
        files: {
          'build/js/main.min.js': 'js/app.js'
        },
        options: {
          transform: [
            'brfs',
          ],
          plugin: [
            ['minifyify', {
              map: 'main.min.map',
              output: 'build/js/main.min.map',
              drop_debugger: true, // passed to uglify.minify as well
              drop_console: true // and this too
            }]
          ]
        }
      }
    }
  });
}
```