refactor: overhaul cli

Functionality is entirely within subcommands now:
- generate: generate a hald clut for external use, replaces not
  providing a subcommand
- apply: same as before
- completions: replaces --completions

This fixes issues where the completion scripts parse the subcommand
as a custom color, and suggests to type the subcommand twice.

Custom colors are also now separated from the command using a `--`

.github/workflows/rust.yml

@@ -45,11 +45,11 @@ jobs:
       - run: |
           chmod +x lutgen
           mkdir output
-          ./lutgen -p catppuccin-mocha -a gaussian-rbf -o output/mocha_gaussian_rbf.png
-          ./lutgen -p catppuccin-mocha -a shepards-method -o output/mocha_shepards_method.png
-          ./lutgen -p catppuccin-mocha -a linear-rbf -o output/mocha_linear_rbf.png
-          ./lutgen -p catppuccin-mocha -a gaussian-sampling -o output/mocha_gaussian_sampling.png
-          ./lutgen -p catppuccin-mocha -a nearest-neighbor -o output/mocha_nearest_neighbor.png
+          ./lutgen generate -p catppuccin-mocha -a gaussian-rbf -o output/mocha_gaussian_rbf.png
+          ./lutgen generate -p catppuccin-mocha -a shepards-method -o output/mocha_shepards_method.png
+          ./lutgen generate -p catppuccin-mocha -a linear-rbf -o output/mocha_linear_rbf.png
+          ./lutgen generate -p catppuccin-mocha -a gaussian-sampling -o output/mocha_gaussian_sampling.png
+          ./lutgen generate -p catppuccin-mocha -a nearest-neighbor -o output/mocha_nearest_neighbor.png
       - uses: actions/upload-artifact@v3
         with:
           name: catppuccin_mocha_hald_cluts

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "lutgen"
-version = "0.7.0"
+version = "0.8.0"
 documentation = "https://docs.rs/lutgen"
 keywords = ["image-processing", "color", "hald-clut", "lookup-tables", "gaussian"]
 description = "A blazingly fast interpolated LUT generator and applicator for arbitrary and popular color palettes."