Replace use of github.com/lunny/html2md

[gibmat]

I'm looking at the feasibility of packaging commento++ for Debian, and while reviewing build dependencies I noticed that this project is using github.com/lunny/html2md, which appears to be dead upstream. I did find github.com/JohannesKaufmann/html-to-markdown, which appears to be an actively maintained HTML to MD converter library. Would it be possible to switch the library that is used by api/domain_import_disqus.go?

[souramoo]

Thanks for looking into this and the spot - I will try and move this dependency over to a more well maintained markdown library shortly!