Add support for definition list

[oscarotero]

Markdown-it has a plugin for definition list based on the Pandoc syntax, which uses the syntax of PHP Markdown Extra.

It would be nice to support this.

Apple
: Pomaceous fruit of plants of the genus Malus in the family Rosaceae.

Orange
: The fruit of an evergreen tree of the genus Citrus.

Dprint (or deno fmt) breaks this code by converting to:

Apple : Pomaceous fruit of plants of the genus Malus in the family Rosaceae.

Orange : The fruit of an evergreen tree of the genus Citrus.