Add `convert.rs`: a SIMD nearest-color search and a rayon-parallel
`nordify_pixels` function. Add `error.rs` with the `NordifyError` type.
Rewrite `lib.rs` to expose `NordifyOptions`, `nordify_image`, and
`nordify_file` as the public API. Rewrite `main.rs` to parse CLI
arguments with `argh` and call into `lib.rs`.
Add `argh` and `thiserror` as dependencies.
Add `Color::euclidian_distance` and `Rgb::lerp`, then use them in a new
`Palette<T>` type whose `expand` method adds colors between close
pairs. Split `palette.rs` into a module, with `NORD_PALETTE` in its own
file.
phundrak