Refactor: directory structure (#31)

* config: update version
* refactor: move source codes to lib directory, Close #30
* refactor: move test code

lib/get-latest-version.js

@@ -0,0 +1,21 @@
+const XMLHttpRequest = require("xmlhttprequest").XMLHttpRequest;
+
+function getLatestVersion() {
+  return new Promise((resolve, reject) => {
+    const xhr = new XMLHttpRequest();
+    const url = "https://formulae.brew.sh/api/formula/hugo.json";
+    xhr.open("GET", url);
+    xhr.send();
+    xhr.onreadystatechange = function() {
+      if (xhr.readyState === 4 && xhr.status === 200) {
+        const result = JSON.parse(xhr.responseText);
+        const latestVersion = result.versions.stable;
+        resolve(latestVersion);
+      } else if (xhr.readyState === 4 && xhr.status !== 200) {
+        reject(`ERROR: got status ${xhr.status} of ${url}`);
+      }
+    };
+  });
+}
+
+module.exports = getLatestVersion;