Add a `!help` command to list out the other commands/usage for the bot

[michael-hawker]

Since we've discovered we can use the bot in DMs, it'd be handy to ask the bot for assistance in how to use it better. 🙂

There could also then be a default response if you send anything to the bot of:

Hi, I'd like to help you but I don't understand what you're asking, use the !help command to find out what I can do for you.

Which would be pretty cool if someone just DMed and said 'Hi' to the bot as a getting started experience.

Typing !help currently would list out something like:

Hi, I'm your friendly Server Companion! Did you know you can also DM me to interact with me privately and not spam channels for project administration?
Available commands:
!app details
!app user add|remove
!getuser ...
!help
!news [/comment "description text"]
!roleinfo ...

Available commands could also be filtered based on user permissions?

This way the bot can be self-documenting as it gets updated vs. having to fill up the announcement channel with all the details. Bonus if !help <command> filters and only shows the usage for that command.

I didn't see a common interface to how commands are implemented, but a 'help' function (or two like one for 'usage' and the other for details when !help <command> is called) would be handy for each command to echo back info.