feat: add siteConfig loader

lib/loader/index.js

@@ -0,0 +1,12 @@
+const path = require('path');
+const loadConfig = require('./config');
+
+module.exports = async function load(sourceDir) {
+  // 1. load siteConfig
+  const siteConfig = loadConfig(sourceDir);
+
+  // 2. extract metadata from markdown files
+  const metadatas = [];
+
+  return null; // todo
+};