Support text alignment for Joybox::UI::Label

Add alignment & dimensions option to Joybox::UI::Label.
For example

    label = Label.new(text: "Text"
      font_size: 20,
      color: Color.new(255, 255, 255),
      position: [Screen.width - LABEL_WIDTH, Screen.height - LABEL_HEIGHT],
      dimensions: CGSizeMake(LABEL_WIDTH, LABEL_HEIGHT),
      alignment: Joybox::UI::Label::TextAlignmentRight
    )