Add support for custom operators inside varnish_purge()

There is no easy way to ban everything except static files (js, css, images), but this can be easily implemented if varnish_purge() would support custom operator for matching patterns.

With this patch applied you will be able to achieve this by using something as simple as that:

$pattern = '(?i)\.(png|gif|jpeg|jpg|ico|swf|css|js|svg)(\?.*)?$';
$pattern = str_replace('\\', '\\\\', $pattern); // escape the pattern
varnish_purge(_varnish_get_host(), $pattern, '!~');

The patch is made against 8.x-1.x branch, but I think it can be easily ported to D7 as well.