Middleware functionality

Issue for discussion about Bot middleware functionality and ideas.

Inspired by [Express middleware functions](https://expressjs.com/en/guide/using-middleware.html).

Express implementation:
```javascript
app.use(function (req, res, next) {
console.log('Time:', Date.now())
next()
})
```

```javascript
Bot.use((chatter, send) => {
// ... some sort of operation with the chatter object, or message string

send() // Allows the message to be sent
})
```

In relation to this, the idea of custom Bot plugins would be cool e.g.
```javascript
const TwitchBotCommands = require('twitch-bot-commands')

// The custom bot module
const commands = TwitchBotCommands("!", {
"test": "command executed!",
"tweet": async chatter => {
const latestTweet = await someAsyncApiRequest(chatter.username)
return `Latest tweet: ${latestTweet}`
}
})

// Instantiate the module
Bot.module(commands)
```

The issue names no repository files, tests, or entry points; start by reviewing the linked Express middleware guide and the proposed Bot.use and Bot.module examples. This is currently a discussion of possible APIs, so done would require an agreed middleware and plugin design before implementation can be scoped.

Written by the indexing model from the issue text.