add rainbow example

examples/screenshot.js

@@ -0,0 +1,18 @@
+'use strict';
+const chalk = require('..');
+const styles = require('ansi-styles');
+
+// Generates screenshot
+for (const key of Object.keys(styles)) {
+	let ret = key;
+
+	if (key === 'reset' || key === 'hidden' || key === 'grey') {
+		continue;
+	}
+
+	if (/^bg[^B]/.test(key)) {
+		ret = chalk.black(ret);
+	}
+
+	process.stdout.write(chalk[key](ret) + ' ');
+}