Add empty partials for custom html in head and footer

As per https://gohugo.io/templates/partials/ "Theme developers may want
to include a few partials with empty HTML files in the theme just so end
users have an easy place to inject their customized content."

Resolves #42

layouts/partials/footer.html

@@ -76,3 +76,4 @@
 {{ end }}
 
 {{ template "_internal/google_analytics.html" . }}
+{{- partial "footer_custom.html" . }}

layouts/partials/footer_custom.html

@@ -0,0 +1,4 @@
+<!--
+If you want to include any custom html just before </body>, put it in /layouts/footer_custom.html
+Do not put anything in this file - it's only here so that hugo won't throw an error if /layouts/footer_custom.html doesn't exist.
+-->

layouts/partials/head.html

@@ -46,4 +46,5 @@
   <link rel="stylesheet" href="https://fonts.googleapis.com/css?family=Open+Sans:300italic,400italic,600italic,700italic,800italic,400,300,600,700,800" />
   <link rel="stylesheet" href="{{ .Site.BaseURL }}/css/pygment_highlights.css" />
   <link rel="stylesheet" href="{{ .Site.BaseURL }}/css/highlight.min.css" />
+{{- partial "head_custom.html" . }}
 </head>

layouts/partials/head_custom.html

@@ -0,0 +1,4 @@
+<!--
+If you want to include any custom html just before </head>, put it in /layouts/head_custom.html
+Do not put anything in this file - it's only here so that hugo won't throw an error if /layouts/head_custom.html doesn't exist.
+-->