Support app_store_category config var for OSX apps.

Allows you to set app_store category configuration value instead of doing this:

```ruby
app.info_plist['LSApplicationCategoryType'] = 'public.app-category.developer-tools'
```

This allows you to specify:

`app.app_store_category = 'public.app-category.developer-tools'`
 ~ or ~
`app.app_store_category = 'developer-tools'`

This value is required to submit apps to the Mac App Store.