Make sure hugo --version prints the version and does *nothing* else.

Any program, when asks to print their version, only prints that, and then
stops.
hugo checks the config (and prints a warning message if not found), and
proceeds to generate the site!
Yet, the user just wanted to check the version.

This patch makes sure hugo stops after printing the version.
Merge pull request #20 from VonC/version

Make sure hugo --version prints the version and does *nothing* else.