Allow other file-extensions

[rietho]

It would be great if markdown-viewer would also allow other file extensions. In my case I like to use Rmarkdown, which uses the extension .Rmd and it would be great to view it with the markdown-viewer.

[ThomasVie]

For me it is a link like the following which does not work:
http://localhost/cgi-bin/dwww/usr/share/doc/unattended-upgrades/README.md.gz
(Debian compresses all larger documentation files. The browser automatically decompresses them.)