script-filters: Allow using script variables in Enable/DisableFilters

To support a non-evil way (if-in-location) of tuning SPDY specific
configuration, allow scripting of Enable/DisableFilters in nginx.conf

Example:
```bash

http {
pagespeed ProcessScriptVariables on;
server {
    pagespeed RewriteLevel PassThrough;
    pagespeed EnableFilters rewrite_images;
    set $spdy_disable "";

    if ($spdy) {
       set $spdy_disable "rewrite_images";
    }
    pagespeed DisableFilters "$spdy_disable";
}
}
```