Add config file including configurable expire times.

This pull request adds a *config.inc.php* which is used to add configurable expire times. For example, a pastebin provider could disable "Never" expiration to avoid clogging up the disk.

Editing index.php and page.html is not a good practice for this, not only because users will need to re-apply changes after each update.

In the future, the config file could be used for further configuration options.

Merging would be highly appreciated!