Load custom JS scripts if page specifies them

Signed-off-by: Danila Fedorin <danila.fedorin@gmail.com>

layouts/partials/head.html

@@ -43,6 +43,12 @@
     {{ end }}
     <link rel="icon" type="image/png" href="{{ $icon.Permalink }}">
 
+    {{ if .Params.custom_js }}
+    {{ range $customJs := .Params.custom_js }}
+    <script src="{{ page.Resources.Get $customJs }}"></script>
+    {{ end }}
+    {{ end }}
+
     {{ if hugo.IsServer }}
     <!-- KaTeX auto-rendering for when we don't have a post-processing step. -->
     <script defer src="https://cdn.jsdelivr.net/npm/katex@0.16.0/dist/katex.min.js" integrity="sha384-X/XCfMm41VSsqRNQgDerQczD69XqmjOOOwYQvr/uuC+j4OPoNhVgjdGFwhvN02Ja" crossorigin="anonymous"></script>