Content editor now supports rendering and editing dl/dd/dt elements.
Creating and editing these elements works in a very similar way to
regular lists.

Type `<dl>` in content editor to insert a new description list.
Press Enter when within a description list to create a new description
term. Press Tab to convert a description term to a description details.
Press Shift-Tab to convert a description details to a description term.
Press Shift-Tab again to convert it to a regular paragraph.

Changelog: added