[new] Allow custom rule directory via -R and --rules

Allow custom rules by using the `-R` or `--rules` option.

![screen shot 2013-09-30 at 5 00 19 pm](https://f.cloud.github.com/assets/887133/1240899/8ab850a4-2a13-11e3-9e05-44cd78d0f297.png)