editor fallback: prefer nvim > vim > vi when $EDITOR is unset

v0.2: add markdown document system for tasks

each task now gets a .md file in ~/.local/share/stask/docs/<list>/<id>.md
with yaml frontmatter (title, date, done). done/undo syncs to frontmatter.
edit opens $EDITOR, show prints doc to stdout. title syncs back to db on edit.
new commands: edit <id>, show <id>. delete/rm cleans up doc files.