fix: google analytics

based on this: mmistakes/so-simple-theme#423

docs/_includes/google-analytics.html

@@ -1,5 +1,5 @@
 <!-- Google tag (gtag.js) -->
-<script async src="https://www.googletagmanager.com/gtag/js?id=G-XE6GY7G848"></script>
+<script async src="https://www.googletagmanager.com/gtag/js?id={{ site.google_analytics }}"></script>
 <script>
     window.dataLayer = window.dataLayer || [];
     function gtag() { dataLayer.push(arguments); }

docs/_includes/scripts.html

@@ -0,0 +1,55 @@
+{%- if site.footer_scripts -%}
+{%- for script in site.footer_scripts -%}
+{%- if script contains "://" -%}
+{%- capture script_path %}{{ script }}{% endcapture -%}
+{%- else -%}
+{%- capture script_path %}{{ script | absolute_url }}{% endcapture -%}
+{%- endif -%}
+<script src="{{ script_path }}"></script>
+{%- endfor -%}
+{%- else -%}
+<script src="https://code.jquery.com/jquery-3.3.1.min.js"
+    integrity="sha256-FgpCb/KJQlLNfOu91ta32o/NMZxltwRo8QtmkMRdAu8=" crossorigin="anonymous"></script>
+<script src="{{ '/assets/js/main.min.js' | relative_url }}"></script>
+<script src="https://use.fontawesome.com/releases/v5.0.12/js/all.js"></script>
+{%- endif -%}
+
+{%- if jekyll.environment == 'production' and site.google_analytics -%}
+{% include google-analytics.html %}
+{%- endif %}
+
+{% if site.mathjax == true or site.mathjax.enable == true %}
+<!-- MathJax -->
+{% capture mathjaxjs %}https://cdn.jsdelivr.net/npm/mathjax@3/es5/{{ site.mathjax.combo | default: "tex-svg" }}.js{%
+endcapture %}
+<script>
+    // http://docs.mathjax.org/en/latest/upgrading/v2.html
+    MathJax = {
+        tex: {
+            tags: "{{ site.mathjax.tags | default: 'ams' }}"    // eq numbering options: none, ams, all
+        },
+        options: {
+            renderActions: {
+                // for mathjax 3, handle <script "math/tex"> blocks inserted by kramdown
+                find: [10, function (doc) {
+                    for (const node of document.querySelectorAll('script[type^="math/tex"]')) {
+                        const display = !!node.type.match(/; *mode=display/);
+                        const math = new doc.options.MathItem(node.textContent, doc.inputJax[0], display);
+                        const text = document.createTextNode('');
+                        node.parentNode.replaceChild(text, node);
+                        math.start = { node: text, delim: '', n: 0 };
+                        math.end = { node: text, delim: '', n: 0 };
+                        doc.math.push(math);
+                    }
+                }, '']
+            }
+        }
+    }
+</script>
+
+<script type="text/javascript" id="MathJax-script" async src="{{ mathjaxjs }}"></script>
+{% endif %}
+
+{%- if page.layout == "search" -%}
+{% include lunr-search-scripts.html %}
+{%- endif -%}