Allow providing JavaScript assets for generated website

Fixes #64, #65

Now users who wish to use Bootstrap dynamic elements (like dropdown menus)
can enable JavaScript either by including required scripts into their
static assets or by referencing their CDN URLs.

By default Alchemy uses no JavaScript - that has not changed.

alchemy/templates/base.html

@@ -59,6 +59,10 @@
       {% include 'include/footer.html' %}
     </div>
   </footer>
+
+  {% for script in THEME_JS_OVERRIDES or () %}
+  <script type="text/javascript" src="{{ url(script) }}" crossorigin="anonymous"></script>
+  {% endfor %}
 </body>
 
 </html>