Add config option to specify path for assets:precompile

Sometimes I want to override ckeditor from the gem with the custom built version from ckeditor.com.
It works fine in the development, but I'm having issues in production, because `Ckeditor::Utils.select_assets` uses `Ckeditor.root_path` as a root folder when it collects the assets for precompile list.

I suggest to add a new option: `asset_precompile_path` which would allow to use `vendor/assets/ckeditor` folder from my app instead. 


Usage:
``` ruby
# Use this hook to configure ckeditor
Ckeditor.setup do |config|
 # Use app folder as a root folder for precompile list in production
  config.asset_precompile_path = Rails.root
end