Remove the hardcoded attributes for the box element

[LagTag]

Related to issue #30. This PR adds the corner rounding (values 0 to 8) and color attribute to the already existing box element.

label = Zebra::Zpl::Label.new(
    :width         => 800,
    :length        => 800,
    :print_speed   => 6,
    :print_density => 5,
    :copies        => 1
)

label_box_test1 = Zebra::Zpl::Box.new :position => [0, 140], :rounding_degree => 3, :line_thickness => 20, box_width: 200, box_height: 200, :color => "G"

label << label_box_test1
label.dump_contents
>>^XA^LL800^LH0,0^LS10^PW800^PR6^FO0,140^GB200,200,20,B,3^FS^PQ1^XZ