harvesthq/chosen — pull request #776 by wadetandy (2012-08-20 23:26:28+00:00)
Title: Add the option to show a 'select all' button for an optgroup when multi-select is enabled.

For a project I'm working on I want users to be able to select all of the options contained in a given optgroup on a multi-select form. This patch allows you to call `$(".chzn-select").chosen({allow_select_group:true})`
to enable this option. With the option enabled, a small "Select All" button is displayed next to the optgroup title.

example.jquery.html has been updated to demonstrate this.

So far I have only added this to the jQuery version of the plugin, as my project uses jQuery, and I wanted to see if it was something you were interested in pulling in before I took the time to add it to the Prototype code, which I'm less familiar with.