add a --bundler option, for forcing rollup or webpack

src/cli/utils/validate_bundler.ts

@@ -0,0 +1,21 @@
+import * as fs from 'fs';
+
+export default function validate_bundler(bundler?: string) {
+	if (!bundler) {
+		bundler = (
+			fs.existsSync('rollup') ? 'rollup' :
+			fs.existsSync('webpack') ? 'webpack' :
+			null
+		);
+
+		if (!bundler) {
+			throw new Error(`Could not find a 'rollup' or 'webpack' directory`);
+		}
+	}
+
+	if (bundler !== 'rollup' && bundler !== 'webpack') {
+		throw new Error(`'${bundler}' is not a valid option for --bundler — must be either 'rollup' or 'webpack'`);
+	}
+
+	return bundler;
+}