Support relative custom config paths

At the moment, you will need to specify the full path to the config
file for it to work:

    sequelize --config /full/path/to/config.json

This path supports relative config paths:

    sequelize --config relative/path/to/config.json

bin/sequelize

@@ -109,7 +109,11 @@ program
   .parse(process.argv)
 
 if(typeof program.config === 'string') {
-  configuration.configFile = program.config
+  if (program.config[0] === '/') {
+    configuration.configFile = program.config
+  } else {
+    configuration.configFile = path.join(process.cwd(), program.config);
+  }
 }
 
 if(typeof program.env === 'string') {