Allows to have named channels

This channels are automatically created at subscription and send, so no previous initialization is needed.

This channels will be childs of a Channel.Named controller at the supervision tree.

Example

iex> alias MOM.Channel
iex> require Logger
iex> mch = Channel.Named.ensure_exists("my-channel")
iex> och = Channel.Named.ensure_exists(:atom)
iex> mch == och
false
iex> Channel.Named.ensure_exists("my-channel")
mch
iex> Channel.subscribe(mch, &Logger.info("Test" ++ (inspect &1))) != nil
true