Adds the optional width & height attributes to the list of options for qr_image via options.size.

Handles size as an attribute for qr_image of type svg.  While size is not directly supported in http://www.w3.org/2000/svg width and height are.  The resulting image style is set to a multiplier of the default viewBox size for display.  If size is not specified the option is set unset (to preserve the behavior from previous versions of qr-image.) The effective display size then becomes 1.