optional arguments

[suntong]

The GNU getopt supports optional arguments:

A long option normally begins with '--' followed by the long option name. If the option has a required argument, it may be written directly after the long option name, separated by '=', or as the next argument (i.e. separated by whitespace on the command line). If the option has an optional argument, it must be written directly after the long option name, separated by '=', if present.

Would cli support optional arguments as well? If true, please make it clear of the following three stages:

0. not specified
1. specified but no argument
2. specified and with argument

Thanks

[mkideal]

optional arguments supported but 0/1/2 stages not reported. I will add API later.