Professional writer? Marked has you covered:
- Scrivener support: view your full Scrivener project as a single document, fully rendered
- Multi-file documents with include syntax and support for mmd_merge and Leanpub formats
- Document statistics, including Fog Index and Readability scoring
- Using the flexible custom processor feature, Marked can even support Fountain, a special Markdown-like syntax for screenwriting
Marked's my favorite writing tool. I use it when writing for the Web because Marked turns (almost) plain text into HTM at a click.
It's wonderful for fiction, too. Whenever I want to read through my novel in Scrivener, I open the draft in Marked.
It's simple, elegant, and perfect for what it does.
(Disclosure: I have no connection to Marked's developers.)