Make `grunt watch:js` include `assets/js/plugins/**/*.js`

If I add JavaScript packages to my `assets/js/plugins/` directory, they must be concatenated, uglified, and added to `scripts.js` to be used in my theme. The `watch:js` task should also note changes to those files and directories.
Merge pull request #1055 from al-the-x/watch-js-plugins

Make `grunt watch:js` include `assets/js/plugins/**/*.js`