Change the API for hiding commands

[pietroalbini]

Based on some feedback received by an user, the API for hiding commands from /help is not that great. Currently you need to define a normal command and then append the name of the command to the bot.hide_commands list. This works, but it would be nicer to hide the command directly from its declaration, like so:

@bot.command("hello", hidden=True)
def hello_command(chat, message, args):
    pass

Current way:

@bot.command("hello")
def hello_command(chat, message, args):
    pass

bot.hide_commands.append("hello")

[pietroalbini]

I'll probably remove the old way to hide commands in the 1.0 release, so I marked this issue as breaking.