Command Alias #91

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

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