Hugo: Add image.Trace method that produces an SVG

[bep]

In the series "bep gets totally distracted", I'm about to add a image.Trace method.

There are several examples of this work in progress here:

https://temp.bep.is/hugotest/section1/trace/

A quick note: Some of the more detailed traces have bigger file sizes than their source, but they compress great (the gzipped version is < 40%) and of course scales better when you go bigger ...

There will be default options and options in config.toml. With the full set of options the usage will be something like this:

{{ $opts := dict "filter" "previtt" "color" "orange" "low" 30 "high" 100 "turdSize" 50 }}
{{ $traced := $img.Trace $opts }} // => SVG resource

I think the default filter will be luma (brightness), which is also the fastest.

/cc @regisphilibert and gang

[regisphilibert]

Looks great! Don't have a concrete use case on my mind but designers must already do!

In the series "bep gets totally distracted".

Distraction is good, building pages from data source is __great__. 😛

[divinerites]

Régis my use case will be some lazy loading for my s**tload photos with small small svg but better looking than the flat grey I use for now.

Great feature bep. Thanks

[bep]

There are several use cases. Maybe someone wants to create a "monochrome blog", where they can possibly also get away with "wget randomcopyrightedimage.jpg" ...