Parse yaml directories as key=value pairs

Hellow I work on https://github.com/snakemake/snakemake/issues/1186

I would like that the YAMLConfigFileParser would parse yaml directories to key-value pairs.


config.yaml
```
default_resources:
  mem_mb: 10
  time:300
```

would be parsed to:
 --default_resources mem_mb=10 time=300

Do you think this is a reasonable feature request? 