Filter to force compile

Changes made to functions or variables registered in PHP, such as when working with the Theme Customizer, don't cause LESS files to be recompiled, so the changes aren't visible in the Customizer preview or the front-end unless a LESS file is updated manually.

A second parameter is available in the `lessc::cachedCompile()` method for forcing the files to be rebuilt, so this filter allows it to be updated when necessary.