Fix issue #225: Some plugins shouldn't force close others.

Added an option called `closeOthersWhenActive` which defaults to true.
When set to false, it will only call the plugin `activate` method instead
of the Picky select/deselect methods.

Removed deprecated method 'forceDeactivate'.
Merge pull request #238 from kdeloach/topic/issue-225

Fix issue #225: Some plugins shouldn't force close others.