add filter to update_option to allow modifications to option prior to saving

[DD32]

At present, Its impossible to filter an option being updated, While there is an action which can be hooked to notify the option has changed, it cannot be modified directly.

Attached is a patch which adds 'pre_update_option_<option_name>', It can be used in combination with a filter on get_option() to seemingly hide various portions of options to the rest of the WordPress admin (or theme, etc), and then add them back in if the option is modified.

[DD32]

(For an example of how i'm using this: ​http://trac.wordpress.org/ticket/4280#comment:39 )

[ryan]

(In [8247]) pre_update_option_* filter from DD32. fixes #7233