Command Alias

Commands can currently only be invoked by their exact name. Bot authors often want a command to respond to multiple names (e.g. `!hello` and `!hi`) without duplicating logic so we would like to allow registering one or more aliases for a command at decoration time.

**Proposed API**
```python
@bot.command(aliases=["hi", "hey"])
async def greet(ctx, name: str):
await ctx.reply(f"Hello {name}!")
```