Create cache directory

src/_data/posts.js

@@ -13,6 +13,10 @@ class PostCache {
 	}
 
 	readCache() {
+		if (!fs.existsSync(".cache")) {
+			fs.mkdirSync(".cache");
+		}
+		
 		if (!fs.existsSync(".cache/wordpress_post_fetch.json")) {
 			this.last_read_date = null;
 			this.posts = {};