Passing options to uglify.minify()

Hi, I opened and closed #47 because I later noticed I didn't need `require('util')._extend` and thought of more things but came back to the simple version.

So this PR makes this possible:

```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',
              compress: { // passed to uglify.minify
                drop_debugger: true,
                drop_console: true
              }
            }]
          ]
        }
      }
    }
  });
}
```