Add support for SVG images in cells

[nmk]

No description provided.

[pointlessone]

The PR doesn't have any description so I'm not sure what the intent is but from the code alone it doesn't look like a good fit for this particular gem. For one, Prawn doesn't have native support for SVG (.pdf.svg call). That probably comes from prawn-svg gem. That being a third-party plugin makes this a good candidate for a plugin for prawn-table.