We discussed this topic after one of the liquid/markdown workshops.
I recently did some testing with Visual Studio Code (https://code.visualstudio.com/) and its “Liquid languages support” extension to design Silverfin templates. This seems to work quite well. It has nice syntax highlighting and even autocomplete (based on already used drops in the open document). It also has an extension for Markdown syntax, but I think this is less useful.
If anybody is using other/better tools, this would be nice to know.