Add namespaces for components and systems for easier file structure

[W3NDO]

When using ecsx, entities are entirely conceptual. I however struggle with keeping everything in my head as the number of components or systems grow. This pull request adds a flag on the systems and components generator --namespace that allows creating the associated files in specific namespaces.

For instance, If we have a game simulation where we have a ship entity(conceptual) with XPosition, YPosition, HealthPoints and Armor components, we could run

  mix ecsx.gen.component HealthPoints integer --namespace Ship
  mix ecsx.gen.component Armor integer --namespace Ship
  mix ecsx.gen.component XPosition integer --namespace Shared
  mix ecsx.gen.component YPosition integer --namespace Shared

and end up with this folder structure:

|-- Components
  |-- Ship
    |-- health_points.ex
    |-- armor.ex
  |-- Shared
    |-- x_position.ex
    |-- y_position.ex

I have added tests to ensure that the components and systems are correctly added to the manager.ex and that the files are correctly generated. Similarly the file content is updated accordingly so that a file like health_points.ex will contain:

  defmodule MyApp.Components.Ship.HealthPoints do
    @moduledoc """
    Documentation for HealthPointsComponent components.
    """
    use ECSx.Component,
      value: :integer
  end