Fix support for CLI options

CLI options are not actually hooekd up to anything. When defining a cli option it would be detected but the option defined in the Gruntfile would be 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.
Merge pull request #19 from middric/command-line-options

Fix support for CLI options