Comment by jamiecollinson

This may help :-)

  (flycheck-define-checker proselint
      "A linter for prose."
      :command ("proselint" source-inplace)
      :error-patterns
      ((warning line-start (file-name) ":" line ":" column ": "
                (id (one-or-more (not (any " "))))
                (message (one-or-more not-newline)
                         (zero-or-more "\n" (any " ") (one-or-more not-newline)))
              line-end))
      :modes (text-mode markdown-mode gfm-mode org-mode))

(More details in my config at https://jamiecollinson.com/blog/my-emacs-config/#syntax-chec...)