Added config to kepp original rails log and set to use custom logger

This patch introduce an additional flag `keep_original_rails_log` to prevent lograge from unsubscribing the original Rails loggers. Also it sets the `Lograge.logger` to a custom one as configured.

By adding the following lines to **application.rb**, we can configure lograge to write to another file and leave the original Rails' log untouched. 
````
config.lograge.keep_original_rails_log = true
config.lograge.logger = Logger.new "#{Rails.root}/log/lograge_#{Rails.env}.log"
````