Allows to configure external commands and provide different arguments
based on runtime values, like language. It extends the current config
of a command to allow a `arg-mappings` section. An example for
ocrmypdf:

```conf
ocrmypdf = {
  enabled = true
  command = {
    program = "ocrmypdf"
### new arg-mappings
    arg-mappings = {
      "mylang" = {
        value = "{{lang}}"
        mappings = [
          {
            matches = "deu"
            args = [ "-l", "deu", "--pdf-renderer", "sandwich" ]
          },
          {
            matches = ".*"
            args = [ "-l", "{{lang}}" ]
          }
        ]
      }
    }
#### end new arg-mappings
    args = [
      ### will be replaced with corresponding args from "mylang" mapping
      "{{mylang}}", 
      "--skip-text",
      "--deskew",
      "-j", "1",
      "{{infile}}",
      "{{outfile}}"
    ]
    timeout = "5 minutes"
  }
  working-dir = ${java.io.tmpdir}"/docspell-convert"
}
```

The whole section will be first processed to replace all `{{…}}`
patterns with corresponding values. Then `arg-mappings` will be looked
at and the first match (value == matches) in its `mappings` array is
used to replace its name in the arguments to the command.