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.
Include groupsConfig.php in MINIFY_CONFIG_DIR.
Explain separate config storage in MIN.txt
Allow optional custom configs to be optional

This change tests first for the existence of the custom config variable and then the existence of each custom config file in turn. If found the directory path to append to that config file is changed to the custom path.

This allows fallback to the standard config when no custom config is present.

Maybe it would be even better to load both - the standard config, then the custom config, but this might be confusing for groupConfig.php. It could work using array merge, but it wouldn't be particularly clear or easy to explain.
Allow full custom path including filename
Change custom path explanation to match full custom paths changes.
Remove filenames from include statements.
Fix index names on config paths array.
Use base for main config key.
Use base for main config key.
Merge pull request #88 from sambauers/master

Allow definition of custom config directory.