Add config option to use include-path in the auto loader

Add a config value to define if the php include-path should be used when generating the autoloader.

This helps if you have a package in the include path (E.G a PEAR package or any custom library) that you want to always have available when using the composer autoloader.
Instead of specifying the ``$loader->setUseIncludePath(true);`` manually with each new project, you can change the global configuration which will then always include the call for the include path