The following exec script will locate all markdown code blocks within the
file and replace them with content fetched using external commands based
on their filetype tag:


Edit , x/\n```@*?```\n/ {
  n/file\n(.*)\n/ { i/\n/; <"head -n 5 {1}"; };
  n/url\n(.*)\n/ { i/\n/; <"curl -s {1}"; };
}


```file
../../../README.md
```

```file
../../../Cargo.toml
```

```url
http://example.com
```

```url
http://wttr.in?format=%c%t\n
```
