Goyo overwrites filetype

I've got a small file in `~/.vim/ftplugin/markdown.vim` to set a few markdown specific settings. In that file I also autostart goyo.

Even though I read a markdown file, the filetype will be reset:

```
:verbose set filetype?
filetype=conf
Last set from /usr/share/vim/vim82/filetype.vim line 2247
```

This does not happen if I start Goyo inside vim. The only solution right now seems to reset the filetype after calling goyo, e.g.

```
[...]
Goyo
set filetype=markdown
```

Reproduce with ~/.vim/ftplugin/markdown.vim configured to autostart Goyo, then inspect :verbose set filetype? and the reported filetype.vim line 2247. Compare this with starting Goyo interactively; done means opening a Markdown file with the ftplugin leaves filetype set to markdown without a manual reset.

Written by the indexing model from the issue text.