A plugin is any module that defines an `init` function that takes a `Bot`
instance as first argument.  
The function can then register any message handlers or timers on the bot,
or add any dependencies it requires.

Example:

```py
from ..functions import reply

HELP = """Responds to your hello.
!hello
"""

def init(bot):
    bot.on_command("hello", reply_world)

async def reply_world(message):
    await reply(message, "world")
```