Allow fine-tuning of SVG output by forwarding keyword arguments

[agnat]

Forwarding the kwargs in SvgFragmentImage.new_image() allows additional fine-tuning of the resulting SVG node. Similarly, adding kwargs to the to_string() method helps to adjust things like the string encoding:

qr = qrcode.QRCode(image_factory=qrcode.image.svg.SvgPathImage)
svg_str = qr.make_image(attrib={'class': 'some-css-class'}).to_string(encoding='unicode')

Both features are mentioned in the README. Let me know what you think...