Fixed -l option

Print an error message when invalid loglevel value is provided
Exit with an error code
Merge pull request #56 from swaeku/master

Inform user when invalid loglevel is provided in command line options
Merge branch 'master' of git://github.com/akrennmair/newsbeuter

* 'master' of git://github.com/akrennmair/newsbeuter:
  Fixed -l option