Render associated images for markdown on github

**Is your feature request related to a problem? Please describe.**

Hackmd.io will pull a markdown file from a github repo but it does not render associated files stored in the same repo, such as any image files meant to be displayed within the markdown document by using the `![]()` syntax.

Hackmd's 'Badge in Github file' button opens a markdown file directly from the github repository but does not render any images linked to from that file, even though those files are available from the same github repository.

**Describe the solution you'd like**
A solution would be to for hackmd to access required files that are available on the same github repository. For example a markdown file and an associated /images folder.

Since the user grants hackmd permission to access an entire repository, this should be possible.