Feature request: niv-sources.toml

[koraa]

Thank you for working on an awesome usability related tool here; this is what we need to make nix more accessible!

I am evaluating ways to simplify dependency handling in Rosenpass. It is hard for me to figure out whether migrating our complex flake.nix to niv could be an option, but maybe it will be the right choice.

In any case, when I saw the sources.json file I noticed two issues:

• The name is kind of non-specific. It would be good to use a more specific name ("niv-sources.json" or "nix-sources.json") so this name will never collide with any other projects using a "sources.json"
• How about a toml file so the file will look prettier in the git history and become easier to hand-edit?