Add banner text to main.min.js (#880)

- Add `---` YAML Front Matter as a workaround to allow the theme gem's version to be overridden locally.
- Adjust page scope in Front Matter default to avoid adding a layout to `main.min.js`
- Fixes #874, Fixes #722

banner.js

@@ -0,0 +1,21 @@
+const fs = require('fs');
+const pkg = require('./package.json');
+const filename = 'assets/js/main.min.js';
+const script = fs.readFileSync(filename);
+const padStart = str => ('0' + str).slice(-2)
+const dateObj = new Date;
+const date = `${dateObj.getFullYear()}-${padStart(dateObj.getMonth() + 1)}-${padStart(dateObj.getDate())}`;
+const banner = `---
+layout:
+---
+
+/*!
+ * Minimal Mistakes Jekyll Theme ${pkg.version} by ${pkg.author}
+ * Copyright ${dateObj.getFullYear()} Michael Rose - mademistakes.com | @mmistakes
+ * Licensed under ${pkg.license}
+ */
+`;
+
+if (script.slice(0, 3) != '/**') {
+  fs.writeFileSync(filename, banner + script);
+}