Support for txt extension, or md.txt

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

I have an extensive library of markdown files in my Dropbox that I would like to edit. I always use .md.txt for the file extension. This way Sublime Text knows these are markdown files, but other applications know to treat the file as plain text. HackMD will not open the files, as they don't end with .md.

**Describe the solution you'd like**

I would like the ability to open and save files with .txt extension, or at least md.txt.

**Describe alternatives you've considered**

Renaming the files to .md is not appealing to me. For me, a large part of the attraction of using Markdown is that the files are plain text, and not some proprietary format. When the file extension reflects this, other applications (e.g. MacOS Finder, Nautilus under Linux) know how to treat the files (e.g. quicklook preview works, files open with a text editor by default).