renamed the undefined plugins variable to selector_plugins

While creating a build by specifying the plugins option as comma separated value it throws an error saying the `ReferenceError: plugins is not defined`

I looked into the Gruntfile.js & found out you forgot to rename the variable in [one of your commits](https://github.com/brianreavis/selectize.js/commit/3a76dcce1ae1438496d12130e86e4616b0089616)