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 ...]

Simple crontab file support

In addition to specifying cronspec-command pairs as positional arguments, the program now attempts to parse /etc/crontab if no positional arguments are provided. If only a single positional argument is specified, it must be a path to a crontab file. However, if the path is not /etc/crontab, which is considered the "system crontab", a crontab file without a user field is expected.

1.0.0

initial impl