Allow preferences that need not be rendered in Special:Preferences

Extensions can use their own interface for user preferences, with the
help of the action=options API.

For example, Universal Language Selector has a different UI to
allow anonymous and logged in users to set language related preferences.

Validation for the preference values is up to the extensions.

Change-Id: I18a5ffb5cc202c59ba76b86cfb63e49010cc1881