renamed the undefined plugins variable to selector_plugins

While creating a build by specifying the plugins option as comma separated value it throws the following error:
```
Loading "Gruntfile.js" tasks...ERROR
>> ReferenceError: plugins is not defined

Running "clean:pre" (clean) task
Verifying property clean.pre exists in config...ERROR
>> Unable to process task.
Warning: Required config property "clean.pre" missing. Use --force to continue.

Aborted due to warnings.
```

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)

@brianreavis