CLI: -f now accepts a file path

The -f option for the CLI will now accept a path to a formatter, as well as the name of a formatter in the ESLint core package. This will allow the end-user to plug in a custom formatter that may not be appropriate for the ESLint core package.

In addition, a bug was fixed in which a failure to find a formatter would incorrectly exit with a 0 (no errors) code.
Merge pull request #172 from iancmyers/feature-formatters-path

CLI: -f now accepts a file path