Can generate the config without installing application through home-manager. Helpful when a package is broken (or not provided) on a specific platform through nixpkgs and needs to be installed through other means but you still can benefit from the declarative configuration.
Adds a program module for [Sapling](https://sapling-scm.com/).
Since Sapling itself is very similar in nature to Mercurial,
`modules/programs/mercurial.nix` was copied to make this module with
the ignore pieces removed (Sapling respects gitignore).
