Adding an environment variable to specify custom configuration file

To ease deployment and integration, the configuration file loaded
by the application is:

• by default, includes/config.php
• if defined, the file located at the OBSIDIAN_CONFIG environment variable

This allows to define the path to a custom configuration file in web server
configuration.

e.g. for nginx:
fastcgi_param OBSIDIAN_CONFIG /path-to-custom/config.php