Add a concrete5 driver that provides the environment (#158)

* Add a concrete5 driver that provides the environment

* Use better directory for detection and allow for accessing separate php files

cli/drivers/ValetDriver.php

@@ -60,6 +60,7 @@ public static function assign($sitePath, $siteName, $uri)
         $drivers[] = 'KatanaValetDriver';
         $drivers[] = 'JoomlaValetDriver';
         $drivers[] = 'DrupalValetDriver';
+        $drivers[] = 'Concrete5ValetDriver';
 
         $drivers[] = 'BasicValetDriver';