Support new template config format

[lovesegfault]

It looks like tinted-theming is moving away from config.yaml files that look like this:

default:
  extension: .ini
  output: colors

And towards this:

base16:
  extension: .ini
  output: colors
  supported-systems: [base16]

base24:
  extension: .ini
  output: colors
  supported-systems: [base24]

Which is breaking base16.nix as it fails to find a default target in the config. We should probably move to looking for a base16 default?

[SenchoPens]

Hi, I'm not planning to break compatibility, there is a way of implementing this but I don't have the resources right now. You can always do it with target = "base16" (see docs for more details)