Multiple configurations

This makes it possible to fetch emails from multiple inboxes.

Here is an example

```ruby
Thread.new do
  config = {
    poll_interval: 10,
    imap: {
      server: 'imap.gmail.com', port: 993, ssl: true,
      username: "some1@gmail.com",
      password: "pwd"
    }
  }
  Mailman::Application.run(config) do
    default do
      p message.subject
    end
  end
end

Thread.new do
  config = {
    poll_interval: 10,
    imap: {
      server: 'imap.gmail.com', port: 993, ssl: true,
      username: "some2@gmail.com",
      password: "pwd"
    }
  }
  Mailman::Application.run(config) do
    default do
      p message.subject
    end
  end
end
```