Implement Help command

[noescape00]

Implement an attribute which we will be able to use on every command and supply there help message. Like that:

        [Command("withdraw")]
        [HelpInfo("Withdraws specified amount of money. Example usage: ....")]
        public Task WithdrawAsync(string address, double amount)
        {
            // TODO
            throw new NotImplementedException();
        }

Using those attributes we will fork a help message and display it when help command is called.

[noescape00]

Implemented