Add more options when generating inputs

[DieracDelta]

We should add more options:

• Allow the specification of the follow attribute for other inputs. I don't intend to hit this use case with #17 . Derive list of inputs by wgetting the input flake url and parsing its inputs. Error out if flake.nix does not exist in root directory.
• Allow path inputs, and the type specification (git or https). spec
• Allow choosing revision