Add module options for setting per-domain properties.

[twoolie]

I've split the options from the previous version into two sets, globalOptions which contains options that can only be set in the [global] section, and domainOptions that can be set per-domain. These are merged together into the himmelblau.settings option which feeds the global section. A new option himmelblau.domains is added which takes an attribute set of domain names to configuration options for that domain, and these feed additional sections in the config file named for each domain.

Addititionally, I replaced the ugly string-concat config generation with nixpkgs' generators.toINI which means if a new option is added in the future, you won't have to touch the config generation code.

All previously working configurations should continue to work as before.

Additionally I made a small cleanup to the Makefile to factor out some commands, and keep named links to the different nix outputs.