remark-images blows up images to 100% width and ignores ppi

[bripkens]

This is a follow up after the discussion in #1509:

gatsby-remark-images is a versatile solution, but it has issues with images <maxWidth. Concretely:

• default solution for getting images into Markdown blows all images up to 100% container size
• images with 144dpi should get written out with a width attribute referencing half their natural width – that's what GitHub does for PNG images at 144dpi uploaded through their interface (Retina Mac saves screenshots at 144dpi and as PNG per default)