Make collapsible 'parent' option independent on '.accordion-group'

[simsalabim]

Currently parent option of collapsible plugin relies on .accordion-group class, which is quite unobvious and discouraging, because according to the documentation is should not.

parent - type(selector) - default(false); If selector then all collapsible elements under the specified parent will be closed when this collapsible item is shown. (similar to traditional accordion behavior)

And this can't be known unless you browse through the source code.
In this pull request I propose a fix.

[mdo]

If this still needs fixing in v3, please resubmit against 3.0.0-wip. Thanks!

[simsalabim]

@mdo, done