v0.8.0: the editor opens any viewable file

Open files, Open folder, drag-and-drop and Open URL on readm3.com/viewer now take any file a browser can show, not only Markdown. Any text file (JSON, code, config, logs, some thirty languages) opens as highlighted, numbered, foldable code and can be edited with e like a note. PDFs and images open in the browser's own viewer. Archives, executables, media and office files are skipped without being read, and so is anything that turns out to be binary. A file with no known extension is sniffed once on import and the language is stored with it, across reloads and sync. The synced-workspace schema accepts any path plus that optional language. PR #13.