Make cover dimming class customisable. (#140)

layouts/partials/site-header.html

@@ -3,7 +3,7 @@
   {{/* Trimming the slash and adding absURL make sure the image works no matter where our site lives */}}
   {{ $featured_image := (trim $featured_image "/") | absURL }}
   <header class="cover bg-top" style="background-image: url('{{ $featured_image }}');">
-    <div class="bg-black-60">
+    <div class="{{ .Site.Params.cover_dimming_class | default "bg-black-60" }}">
       {{ partial "site-navigation.html" .}}
       <div class="tc-l pv4 pv6-l ph3 ph4-ns">
         <h1 class="f2 f-subheadline-l fw2 white-90 mb0 lh-title">