Support for array of files in `pluginHelpers`

It enables to include plugins' file into more than one output files.
E.g Handlebars were not included in `vendor-dev` with a following config
```
javascripts:
  joinTo:
    'javascripts/app.js': /^app/
    'javascripts/vendor.js': /^vendor/
    'javascripts/vendor-dev.js': /^vendor(\/|\\)(?!prod-)/
```
Now you can add 
```
pluginHelpers:
  ['javascripts/vendor.js', 'javascripts/vendor-dev.js']
```
to include it in both of the files.