[Fix] Write status messages to stderr instead.

[ycd]

Don’t write status messages to stdout.

• Currently using pipes with --append=false writes

   ~$ toc -p <(curl https://raw.githubusercontent.com/ycd/toc/main/README.md 2>&-) --append=false > toc.md
   ~$ cat toc.md

    - [Table of Contents](#table-of-contents)
        * [Usage](#usage)
        * [Installation](#installation)
            * [Packages](#packages)
            * [Downloads](#downloads)
            * [Installation from source](#installation-from-source)
                * [Unix/Linux](#unixlinux)
        * [Contributing](#contributing)
        * [Licence](#licence)
    ✔ Table of contents generated successfully.
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ <- This shouldn't be included