Add a pre_set_theme_mod_$name filter to set_theme_mod().

This is modeled after pre_update_option_$option in update_option() and pre_set_transient_$transient in set_transient().

props obenland.
fixes #14721.


git-svn-id: https://develop.svn.wordpress.org/trunk@27393 602fd350-edb4-49c9-b593-d223f7449a82