Allow relative path for markdown images

[yathomasi]

Currently, gatsby-remark-relative-images have been providing this feature which is being used by a huge numbers of developers.

So, I guess this should be considered to make this part of the existing gatsby maintained plugin gatsby-remark-image or separately. It's because the package is no longer maintained for a while and has been detected with a long list of security issues.

cc: @LekoArts