Provide the ability to disable LESS compilation on all requests

[joehoyle]

@sanchothefat I added the ability to override the less compiling all the time, so we can do:

add_filter( 'pre_option_wp_less_always_compile_less', function() {
    return 0;
} );

This means only the vars and version of the LESS file is used to check if the file should be regenerated. I make this off your md5 pull-request as I think we want that too still.