Fix support for CLI options

CLI options are not actually hooked up to anything. When defining a cli option it is detected but the option defined in the Gruntfile is used instead. This pull request adds support so that cli options override gruntfile options. It also fixes 2 other issues I noticed:

1. It was not possible to set severity or warning severity to 0 as this evaluates as falsey so the option was never passed to phpcs
2. Although its in the README.md file error-severity was not defined.