add PLAUSIBLE_PREFIX support to Helpers.php

[semanticfire]

Allow a different filename in proxy configuration

related to issue #228
Some ad blockers bluntly block anything that contains plausible Making the $file_name configurable together with the proper rewrite rule solved this.

wp_config.php should contain a line like

define('PLAUSIBLE_PREFIX','stats')

my nginx config now reads:

 location ~ /js/stats.(?<target>(.*?)).js {
        proxy_pass https://myplausibel.com/js/plausible.$target.js?ver=2.1.4;
        proxy_set_header Host myplausible.com;
}