Support default Phoenix repo configuration.

[thruflo]

When you generate a new Phoenix app, it sets the database connection details in dev to something like:

# Configure your database
config :foo, Foo.Repo,
  username: "postgres",
  password: "postgres",
  hostname: "localhost",
  database: "foo_dev",
  stacktrace: true,
  show_sensitive_data_on_connection_error: true,
  pool_size: 10

This then fails with:

config :phoenix_sync,
  mode: :embedded,
  electric: [
    repo: Conductor.Repo
  ]

Because the :port isn't configured:

** (Mix) Could not start application phoenix_sync: Phoenix.Sync.Application.start(:normal, []) returned an error: shutdown: failed to start child: Electric.StackSupervisor
    ** (EXIT) %NimbleOptions.ValidationError{message: "required :port option not found, received options: [:username, :password, :hostname, :database, :sslmode]", key: :port, value: nil, keys_path: [:connection_opts]}

The workaround is to provide the port in the repo config but it would be nice to default this to 5432.