Allow definition of custom config directory.

To allow "clean" checkouts of Minify we could use the possibility to define a custom config directory outside the minify directory.

Using this patch a user can define MINIFY_CUSTOM_CONFIG_DIR on a new page outside the minify directory and then include minify's index.php on that page and point their rewrites etc. there instead.

The config.php (and config-test.php) could be copied to the custom directory and edited without causing svn:external/git:submodule conflicts.