move config into single file, add errors to help people migrate

src/core/read_template.ts

@@ -0,0 +1,17 @@
+import * as fs from 'fs';
+import { locations } from '../config';
+
+export default function read_template() {
+	try {
+		return fs.readFileSync(`${locations.src()}/template.html`, 'utf-8');
+	} catch (err) {
+		if (fs.existsSync(`app/template.html`)) {
+			throw new Error(`As of Sapper 0.21, the default folder structure has been changed:
+  app/    --> src/
+  routes/ --> src/routes/
+  assets/ --> static/`);
+		}
+
+		throw err;
+	}
+}