feature: document exit codes in help text

[elliot-nelson]

SUMMARY

I was recently working on an internal tool that needed some specific exit codes to play well with other tools, and I tried my hand at documenting the exit codes in the help screen itself. I took a screenshot:

Would it be useful if a user could add information about exit codes directly to their help screen with an API method?

Here's the code I used to add this info (I cheated and used the epilogue, and made calls into my style object for the styling):

    function formatHelpTextHash(header, hash) {
        const style = require('sywac-style-chunky');
        const keyLength = Math.max(...Object.keys(hash).map(key => String(key).length));

        return [
            style.group(header),
            ...Object.keys(hash).map(key => `  ${style.usageOptionsPlaceholder(key).padEnd(keyLength)}  ${hash[key]}`)
        ].join('\n');
    }

    sywac.epilogue(formatter.formatHelpTextHash({'Exit Codes:', {
            0: 'success',
            1: 'unexpected error',
            2: 'one or more nodes over the --consecutiveReboots limit',
            3: 'one or more nodes exactly at the --consecutiveReboots limit'
    });

DETAILS

Add an optional help text API method exitCodes to describe a list of exit codes and their meanings. Does not appear in help screen unless specified. Formatted using your styling... somehow. (I used .usageOptionsPlaceholder just because I liked the look in my case, but it could be more appropriate to create a specific style for this. Or we could pick something like .hints.)