logger: add --skip-empty to prevent logging empty lines

logger: add --skip-empty to prevent logging empty lines

Empty log messages are generally considered useless. This option
enables to turn them off when processing files (including stdin).

[kzak@redhat.com: - rename --skip-empty-lines to --skip-empty,
                  - add the option to getopt_long(),
                  - add the option to bash-completion]

Signed-off-by: Karel Zak <kzak@redhat.com>