Retina 2x <lasso-img>

[oxala]

#151

• Added support to add "2x" retina images using srcset="./path.png" in ;
• Updated tests;

@patrick-steele-idem

After a couple of attempts to do it and process srcset={"2x": "./path.png"} - i failed and fallen-back to a simpler solution.

The problem i couldn't resolve was that marko/compiler/Builder didn't like me passing array instead of a single src here (https://github.com/lasso-js/lasso/compare/master...oxala:Lasso-img-Retina?expand=1#diff-bceb7acc917818bc45d3fb926f0dd232R55) and was crashing (with message like arg should be string, Node or null). Even though you said i need to allow arrays inside taglib/helper-getImageInfo.js, i think it's not the case, since it doesn't even get there, the Builder error comes before that.

Solution below adds processed srcset="./path.png to the result image in format srcset="/static/.../path.png 2x", so >2x are at least presented with 2x image. If there is not srcset on the lasso-img, then of course no srcset presented in the output.

P.S And of course if this implementation can be improved (or if it's complete rubbish) - i would love to hear it.