Add a debug option to output the i18n-keys

[tordans]

For me as a translator this gem's magic is just too much. I spend way too much time searching for the right i18n-key for my flash-message-usecase.

It would be awesome to have a debug-option that we can turn on in our dev+staging-mode.
This option should output something like this:

<div class="notice flash">
Your Current Flash Message
<!-- 
possible locale_flash keys:
  controllers.admin.users.create.flash.notice # blank
  controllers.admin.create.flash.show.notice # Your Current Flash Message
  controllers.admin.create.flash.notice # Some fallback text
  controllers.admin.flash.notice # blank
  controllers.flash.create.notice # blank
  controllers.flash.notice # Some fallback text
-->
</div>

This is the tree of all possible translations. Together with the translation-content.
This way I just need to inspect the flash message to know where it came from and know how to edit it.

Tobias