Fix truncated extension using Plain strategy

The actual `Phrozn/Site/View/OutputPath/Plain.php` uses `rtrim` to remove the file extensions.

The problem is that if you configure a permalink like `test.xml` the output is `test.x` because of `rtrim` (similar to `test.txt` becomes `test.tx`. Using the `preg_replace` fix this bug.

I hope that is useful.

Walter