Additional stream prefix per aggregate

[slashdotdash]

Add support to command router to allow further specification of an aggregate identity and additionally an identity prefix.

1. Allow the aggregate identity prefix to be specified during command registration:

   defmodule BankRouter do
     use Commanded.Commands.Router
   
     dispatch [OpenAccount,DepositMoney,WithdrawMoney] 
       to: BankAccount, 
       identity: :account_number,
       identity_prefix: "bank-account-"
   end

2. Allow a function to be used as the identity option; you construct the identity as required from the command being dispatched:

   defmodule BankRouter do
     use Commanded.Commands.Router
   
     dispatch [RegisterUser] 
       to: User, 
       identity: fn %{uuid: uuid} -> "user-#{uuid}" end
   
     dispatch [OpenAccount,DepositMoney,WithdrawMoney] 
       to: BankAccount, 
       identity: &BankRouter.account_identity/1
   
     def account_identity(%{account_number: account_number}), do: "bank-account-#{account_number}"
   end

3. Allow an aggregate identity to be specified for all its commands, providing an optional prefix:

   defmodule BankRouter do
     use Commanded.Commands.Router
   
     identify BankAccount, 
       by: :account_number, 
       prefix: "bank-account-"
   
     dispatch [OpenAccount,DepositMoney,WithdrawMoney] to: BankAccount
   end

Closes #77.