Make it possible to specify non-durable exchanges

[lukyanov]

Description

Currently it is not possible to declare exchanges as non-durable (durable: false). This PR adds an option to do this.
Non-durable exchanges (and queues for the same matter) is quite useful in development when restarting the RabbitMQ server clears everything.

[vorce]

Thanks a lot for the contribution @lukyanov

Got an idea on how to make it easier to use all of amqp's options

[vorce]

I suggest we change the API for declare_exchange (in a backwards compatible way) so we can use all options supported by amqp. Maybe something like:

def declare_exchange(chan, exchange, type \\ :topic, options \\ [durable: true])

def declare_exchange(_chan, :default, _type, _opts), do: :ok

# For backwards compatibility
def declare_exchange(chan, {type, exchange}, _type, _opts) do
  declare_exchange(chan, exchange, type, durable: true)
end

def declare_exchange(chan, exchange, type, options) do
  Exchange.declare(chan, exchange, type, options)
end

what do you think about it @mkorszun @lukyanov ?