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.