Only attempt to parse config file if it is specified by flag (#169)

Previously, the behavior was to search for the config file in a variety of
locations. Now we only attempt to load a config file if it has been explicitly
specified with `-c` or `--config`.

Fixes #165.

cli/root.go

@@ -46,7 +46,7 @@ wish to override the default value.
 `,
 	}
 
-	rootCmd.PersistentFlags().String("config", "", "Path to a configuration file")
+	rootCmd.PersistentFlags().StringP("config", "c", "", "Path to a configuration file")
 
 	return rootCmd
 }