Add `extraConfig` option for SLiM

so that various configuration options can be set without having to expose every single configurable parameter
Ah, okay ! Didn't find that type in the manual, was looking for something like that. But maybe concatenation does not work for slim conf. not really sure, but from what i know it looks like a bunch of space separated key value pairs on different lines.
```
option1         value1
option2         value2
option3         value3
```
( From what I see here, https://wiki.archlinux.org/index.php/SLiM )