DuckPAN Server - Add support for <svg>, <canvas>, etc

Previously HTML::Treebuilder was throwing away `<svg>` and `<canvas>` elements as they aren't part of the known HTML Tagset.

I've turned on the flag to allow "unknown_elements" which causes the parser to keep these unknown elements rather than throw it away.

This is in regards to a recent Goodie Pull Request: https://github.com/duckduckgo/zeroclickinfo-goodies/pull/464#issuecomment-47450394

//cc @russellholt @jagtalon