Add new Javascript dependancies:

Underscore: http://underscorejs.org/
Showdown: https://github.com/coreyti/showdown
OpalDown: An OPAL flavour of markdown.

We add a pre-processing step to the markdown parse/render cycle,
whereby single line breaks become "  \n" - which Markdown will
happily consider to be a paragraph.
Add markdown-details directive.

Watch for changes to item.details. When changed, render our element's
contents as the transformed result of Opal Flavoured Markdown.
Remove debug logs.