Support for multiple cronfiles

Usage has been extended:

# loads jobs from /etc/contab
croncmd

# jobs defined as positional arguments
croncmd [flags] 'CRON SPEC' 'COMMAND' ['CRON SPEC' 'COMMAND' ...]

# positional arguments as globs to find multiple cronfiles
croncmd [flags] -crontabs /etc/crontab '/etc/cron.d/*' [other/globs ...]