Add support for loading custom formatter from package

docs/user-guide/usage/options.md

@@ -88,7 +88,7 @@ Options are:
 - `unix` - generates messages like a C compiler, so that tools like Emacs' _Compilation mode_ can hyperlink them
 - `verbose` - extends `string` to include a list of checked files and a tally for each rule
 
-The `formatter` Node.js API option can also accept a function, whereas the `--custom-formatter` CLI flag accepts a path to a JS file exporting one. The function in both cases must fit the signature described in the [Developer Guide](../../developer-guide/formatters.md).
+The `formatter` Node.js API option can also accept a function, whereas the `--custom-formatter` CLI flag accepts a path (either a filesystem path or a dependency) to a JS file exporting one. The function in both cases must fit the signature described in the [Developer Guide](../../developer-guide/formatters.md).
 
 ## `cache`
 

lib/__tests__/__snapshots__/cli.test.js.snap

@@ -94,6 +94,7 @@ exports[`CLI --help 1`] = `
     --custom-formatter
 
       Path to a JS file exporting a custom formatting function.
+      The file can either be a filesystem path, or a file to load from a dependency.
 
     --quiet, -q
 

lib/__tests__/cli.test.js

@@ -333,4 +333,18 @@ describe('CLI', () => {
 
 		expect(process.stdout.write).toHaveBeenCalledTimes(0);
 	});
+
+	it('--custom-formatter', async () => {
+		await cli([
+			`--custom-formatter=${fixturesPath('formatters/customFormatter')}`,
+			'--config',
+			fixturesPath('default-severity-warning.json'),
+			fixturesPath('empty-block.css'),
+		]);
+
+		expect(process.stdout.write).toHaveBeenCalledTimes(1);
+		const output = process.stdout.write.mock.calls[0][0];
+
+		expect(output).toBe('Custom formatter reports 1 warning(s).');
+	});
 });

lib/__tests__/fixtures/formatters/customFormatter.js

@@ -0,0 +1,3 @@
+module.exports = function customFormatter(messages) {
+	return `Custom formatter reports ${messages.length} warning(s).`;
+};

lib/cli.js

@@ -179,6 +179,7 @@ const meowOptions = {
       --custom-formatter
 
         Path to a JS file exporting a custom formatting function.
+        The file can either be a filesystem path, or a file to load from a dependency.
 
       --quiet, -q