Add support for SVG patterns

[ashiguruma]

Many data graphics libraries use an SVG element instead of a Canvas element. It would be helpful if the library provided a way to choose what type of elements to output.

Possible API Formats

// global format
pattern.format('svg');
pattern.draw('circle', '#333');

// pattern specific format
pattern
  .format('svg')
  .draw('circle', '#333');

// default format
pattern
  .draw('circle', '#333');

[JewelsJLF]

+1 we're trying to export a canvas chart using patternomaly to SVG using canvas2svg, but it's not working