Agregado boton git history cuando ingresas por primera vez a un archi…

…vo. Fix #2

extension/main.js

@@ -1,10 +1,44 @@
-let url = chrome ? document.URL : window.location.href;
-const auxUrl = url.split('/');
-auxUrl[2] = 'github.githistory.xyz'
-url = auxUrl.join('/');
+this.isButtonInserted(document.URL);
+let oldUrl = document.URL;
+// It's necessary because git use pjax to refresh views
+// https://stackoverflow.com/questions/3522090/event-when-window-location-href-changes#46428962
+window.onload = () => {
+  let bodyList = document.querySelector('body');
+  const observer = new MutationObserver(mutations => {
+    mutations.forEach(mutation => {
+      if (oldUrl != document.URL) {
+        if (isButtonInserted(document.URL)) {
+        oldUrl = document.URL;
+        }
+      }
+    });
+  });
 
-const buttonGithubHistory = document.createElement('a');
-buttonGithubHistory.innerHTML = 'Open in Git History';
-buttonGithubHistory.setAttribute('class', 'btn btn-sm BtnGroup-item');
-buttonGithubHistory.setAttribute('href', url);
-document.getElementById("raw-url").parentNode.appendChild(buttonGithubHistory);
+  let config = {
+    childList: true,
+    subtree: true
+  };
+  observer.observe(bodyList, config);
+};
+
+function isButtonInserted(url) {
+  if (/https:\/\/github\.com\/[a-zA-Z0-9-_]*\/[a-zA-Z0-9-_]*\/blob\/.*/.test(url)) {
+    const auxUrl = url.split('/');
+    auxUrl[2] = 'github.githistory.xyz';
+    url = auxUrl.join('/');
+    const buttonGithubHistory = document.createElement('a');
+    buttonGithubHistory.innerHTML = 'Open in Git History';
+    buttonGithubHistory.setAttribute('class', 'btn btn-sm BtnGroup-item');
+    buttonGithubHistory.setAttribute('href', url);
+    try {
+      document
+        .getElementById('raw-url')
+        .parentNode.appendChild(buttonGithubHistory);
+      return true;
+    } catch (error) {
+      return false;
+    }
+  } else {
+    return false;
+  }
+}