originally sourced from https://github.com/Ovski4/jekyll-tabs.

changes:

• template.erb adapted to match the pre-existing tab html structure for docs.scala-lang.org
• tabs do not use secure random uuid as the id, the tabs name parameter is used instead.
• for the tabs block, add an optional second class='foo bar' parameter to allow custom classes for the tabs.
• for the tab block, reorder the parameters: the tab label comes first, followed by the name of the parent tabs.
• addition of a third defaultTab attribute for tab, which defaults to the final tab if not set.