Allow widget sidebars to be loaded open or closed via filtering

[silb3r]

This could be useful to allow certain sidebars to always load open or closed.

[silb3r]

Add the "registered_sidebar_wrap_class" filter to enable filtering of sidebars' $wrap_class

[welcher]

You may want to consider putting the classes into an array then passing that array to the filter. This will allow for much easier manipulation in the filter. See new patch to see what I mean.

Also, the filter needs some inline docs - ​https://make.wordpress.org/core/handbook/best-practices/inline-documentation-standards/php/#4-hooks-actions-and-filters

[iamfriendly]

Refreshed the patch and added docs

[iamfriendly]

Refreshed the patch and added docs for the new filter.