To complete my complement, on all platforms, of markdown-based editors with folding, I have also purchased Editorial. This application turns my old iPad 2 into quite a powerful editing program. Like Haroopad and Smartdown, it allows me to pull things from Dropbox, edit the markdown files, then send or save them as plain text, HTML, and even PDF. All of them share a design aesthetic I like: very clean, very similar in function. Editorial also has a very handy touchscreen way to drag paragraphs around. Until now, on the iPad, I really haven't seen anything that did quite everything I wanted. Editorial's basic functions are clear and powerful; the built in Python scripting could scale to very complex things. But I don't need to go there, yet.
So now that I'm done tinkering with my tools, and have established a master file on Dropbox available from Windows, Linux, and iOS, all I have to do is write.