Added allowed_content and custom editor css support to configuration 

After update to ckEditor 4.1 my custom HTML containing Elements like <section> was detroyed.

However setting `allowed_content` to `true` avoids breaking my markup.
The other change allows users to style ckEditor contents with a CSS file from their theme.

This requieres stylesheetparser plugin for ckEditor which is not shipped with Bolt.cm ATM