0.4.0: "Let's make help harder for API users"

• Help command has a new alias, man, because I'm a nerd.
• API BREAKING CHANGE: Everything related to command help has been removed from the Command class. Instead, help is now done by setting custom properties on instances of the class and having the help command handle them itself.
• API: The client now has a new property, mentionPrefixRegExp, which is a RegExp used to test messages for a mention of the bot with it rather than re-creating it every time it's needed.
• API: The client now has a static property which references itself, which means you can now do const {Client, Command} = require('Yuuko').