use image paths relative to md file

[prozessorkern]

Description

we use this plugin to include readme.md files into the sphinx documentation. In this usecase used images are referenced relative to the *.md file - currently sphinx will not be able to find those files.

With this fix sphinx will search for images relative to the md file.

[michimussato]

Merge please.

[amyreese]

Would be nice to include some test cases or expected inputs/outputs.