Please add support for Markdown

[KathyReid]

Firstly, the project is amazing!
Thank you for all your efforts.

I do a lot of documentation and writing work in Markdown, is it possible to support Markdown?
If you point me to documentation on how to add support, I'm very happy to have a go at a PR.

Markdown is supported by Tokei

The reason I think Markdown is not supported yet is that I ran it on a repo that is primarily Markdown, and it stated:

kathyreid@kathyreid-ubuntu-X510UQ:~/Dropbox/ANU/CECS8001-Lab/kathy-sensebook-semester2$ onefetch -d .
Error: Could not find any source code in this directory

Kind regards, and thanks again for your efforts.

[o2sh]