Support dumping custom paths instead of just `/` about dconf2nix

[austinbutler]

@gvolpe that looks great, thanks! At the very least it at least makes it more clear that it's expecting the root by default, which should help dummies like myself in the future 😉.

[austinbutler]

Actually I see what's happening, and it's a me problem 🤕. dconf2nix can't know where to place the dump when it's not at the root level. Not sure what I was expecting...

I suppose the only suggestion I could offer is a warning or something in the README about dumping keys other than root.

[gvolpe]

I think we could add support for dumping keys other than /, maybe something like:

$ dconf dump /com/gexperts/Tilix/ | dconf2nix --root /com/gexperts/Tilix/ > dconf.nix

Though, I'm not sure how common this use case may be.

[gvolpe]

@austinbutler have a look at #23 and let me know if that looks good to you or you have other suggestions :)