Adding "regex" option for replace parameter.

Hello!

We've been using your plugin for a while on one of our client websites. It's been working great! However, we had a slight problem with file downloads being inconsistent when we instituted a url-rewrite rule and changed the 'site name' configuration option in Expressionengine. Basically, our site had 2 common domains www.sitename.com and sitename.com.

In order to fix this I simply added a "use_regex" option to the plugin that allowed us to remove both instances from the file_path variable and get a clean relative path.

I thought some others might run into a similar problem, so I'd share. Hope you or someone else finds this useful. :)