Definitionlist plugin

A simple definition list plugin.

Provides two buttons, ToggleDefinitionList and ToggleDefinitionItem, and two editor commands of the same names.

Also styles definition lists in the editor for easy editing and changes the behavior of the enter key (splits definition lists) and tab key (toggles between dd and dt) in definition lists.

Known issues:
- no way to easily merge consecutive definition lists.
- ToggleDefinitionItem works on only one item at a time
- Selection / cursor position is not always restored satisfactorily

Also, please note that this is my first pull request on any project ever. Please forgive any mistake or faux pas I may have committed.