Introduce wpcf7_load_css function and filter hook. Allows to control on/off CSS loading from theme's functions.php. The filter can override WPCF7_LOAD_CSS constant.

git-svn-id: https://plugins.svn.wordpress.org/contact-form-7/trunk@940375 b8457f37-d9ea-0310-8a92-e5e31aec5664