Add macOS built-in color picker capability

`osascript` is the built-in automation program in macOS.

The method of triggering the macOS color picker from `osascript` is documented here:
https://developer.apple.com/library/archive/documentation/LanguagesUtilities/Conceptual/MacAutomationScriptingGuide/PromptforaColor.html#//apple_ref/doc/uid/TP40016239-CH86-SW1

There is a weirdness of `osascript` in that `console.log` only writes to
`stderr` regardless of the documented "write to stdout" flag.
A workaround was found here and linked in the relevant comment:
https://apple.stackexchange.com/a/278395

The picker is only attempted when `#[cfg(target_os = "macos")]`.
However it's still listed in the "Supported tools" help text because
`clap` make it somewhat hard to customize that help text (e.g.
generating from the list of tools) because it does not take ownership of
strings. Related: clap-rs/clap#1041

src/cli/cli.rs

@@ -175,7 +175,8 @@ pub fn build_cli() -> App<'static, 'static> {
                   - grabc (https://www.muquit.com/muquit/software/grabc/grabc.html)\n  \
                   - colorpicker (https://github.com/Jack12816/colorpicker)\n  \
                   - chameleon (https://github.com/seebye/chameleon)\n  \
-                  - KColorChooser (https://kde.org/applications/graphics/org.kde.kcolorchooser)")
+                  - KColorChooser (https://kde.org/applications/graphics/org.kde.kcolorchooser)\n  \
+                  - macOS built-in color picker")
         )
         .subcommand(
             SubCommand::with_name("format")

src/cli/colorpicker.rs

@@ -61,9 +61,27 @@ struct ColorPickerTool {
     version_output_starts_with: &'static [u8],
 }
 
-/// Run an external X11 color picker tool (e.g. gpick or xcolor) and get the output as a string.
+/// Run an external color picker tool (e.g. gpick or xcolor) and get the output as a string.
 pub fn run_external_colorpicker() -> Result<String> {
     let tools = [
+        #[cfg(target_os = "macos")]
+        ColorPickerTool {
+            command: "osascript",
+            // NOTE: This does not use `console.log` to print the value as you might expect,
+            // because that gets written to stderr instead of stdout regardless of the `-s o` flag.
+            // (This is accurate as of macOS Mojave/10.14.6).
+            // See related: https://apple.stackexchange.com/a/278395
+            args: vec!["-l", "JavaScript", "-s", "o", "-e", "
+                const app = Application.currentApplication();\n
+                app.includeStandardAdditions = true;\n
+                const rgb = app.chooseColor({defaultColor: [0.5, 0.5, 0.5]})\n
+                  .map(n => Math.round(n * 255))\n
+                  .join(', ');\n
+                `rgb(${rgb})`;\n
+            "],
+            version_args: vec!["-l", "JavaScript", "-s", "o", "-e", "'ok';"],
+            version_output_starts_with: b"ok",
+        },
         ColorPickerTool {
             command: "gpick",
             args: vec!["--pick", "--single", "--output"],