allows a handling function to be associated with a command

[bcoe]

allows a handling function to be associated with a command see #156

you can now optionally provide a third argument to command, which represents a handler for the function.

This command will get an instance of yargs passed into it, which can then be used to parse arguments specific to the command:

        yargs(['snuh', '-h'])
          .command('blerg', 'handle blerg things', function (yargs) {
            yargs.command('snuh', 'snuh command')
              .help('h')
              .argv
          })
          .help('h')
          .argv

I found that I tend to write boiler-plate code for a lot of my command-line applications, for facilitating this functionality.