Allow to include special hackmd links in github READMEs

**Is your feature request related to a problem? Please describe.**
A README on github supports markdown, but is quite limited, e.g. no mermaid, no slides mode, etc...

**Describe the solution you'd like**
I'd like to include a link to hackmd.io with a URL param to one or more markdown files in my github repository, so when somebody clicks and opens it, hackmd fetches the content from github and displays it in hackmd (slide mode or whatever mode) ...without the necessity for a browser extension.

**Describe alternatives you've considered**
- telling people to not install the browser extension
- writing my stuff in markdown and then having an empty README.md on github with a single link to my markdown document, but that removes the markdown content from my repository :/