script: Add alignment argument to Image.Text

script: Add alignment argument to Image.Text

Enables scripts to choose the text alignment they want with a seventh
argument to Image.Text API:

    new_image = Image.Text("Hello\n"
                           "Plymouth aligned world",
                            1, 1, 1, 1,
                           "DejaVu Bold,Italic 18",
                           "center");

This argument can be ignored, the default being left aligned.
Possible values are "left", "center", or "right".

http://lists.freedesktop.org/archives/plymouth/2012-August/000676.html