feat: add executable for npx. See swordev#59

Usage example: `npx merge ./foo.json ./bar.json`.

merge-cli.mjs

@@ -0,0 +1,49 @@
+#!/usr/bin/env node
+import mri from 'mri';
+import fs from 'fs';
+
+import { merge, recursive } from "./lib/index.js";
+
+const args = mri(process.argv.slice(2), {
+  alias: {
+    s: 'shallow',
+    h: 'help',
+  },
+  boolean: ['shallow'],
+});
+
+const inputFiles = args._;
+
+// Why to avoid `process.exit()`: https://stackoverflow.com/a/37592669/521957.
+if (args.help) {
+
+  const help = `\
+npx merge
+  [--shallow or -s] # If merge is shallow then recursion won't be used.
+  [--help or -h]
+  [file1.json file2.json ...]
+`;
+  process.stdout.write(help);
+
+} else if (!inputFiles.length) {
+
+  console.log({});
+
+} else {
+
+  const objects = inputFiles.map(
+    (path) => {
+      const json = fs.readFileSync(path, 'utf-8');
+      return JSON.parse(json);
+    }
+  );
+  const ifToClone = false;
+  let merged;
+  if (args.shallow) {
+    merged = merge(ifToClone, ...objects);
+  } else {
+    merged = recursive(ifToClone, ...objects);
+  }
+  console.log(merged);
+
+}