Add SVG support

[puzrin]

• Need streaming parser
  • need quick fail (if data does not starts with <)
  • avoid binary to text conversion before quick fail phase
  • may be sax could help (after quick fail passed)
• Design API to be compatible - SVG can have floating & not numeric size
• Should we allow to disable SVG mode?

[puzrin]

Preliminary API suggestions:

• Add result.wUnits & result.hUnits (px, em, ...). px everywhere except SVG.
• Allow floats for result.width & result.height
• Add options param (second) with allowed types list:
  • { types: [ 'svg', 'tiff', 'png', 'webp', ...] }
  • "jpg png webp"
• ? Add .default() method: .default({ types: [ 'svg', 'tiff', 'png', 'webp', ...] })