Improve support for SVG. Fix issue #802 - SVGs without width/height attributes. Also add support for SVG sanitizing via customizable whitelist.

Sorry about the whitespace changes in these files. Must be something I have set in ST3. I am sure you know, but it's easy to hide whitespace differences by appending ?w=1 to the Github URL.

I am sure you'll have some better ideas on how to achieve some of these things, but this seems to be working well. I'll leave some more comments about what I have done under Issue #802 .