feat: run PostCSS on vendor CSS (#2644)

Currently, PostCSS can optionally be run on the main CSS bundle. This commit allows PostCSS to also be run on the vendor CSS. This can greatly reduce the size of the vendor CSS bundle when combined with PurgeCSS since most of the Font Awesome CSS is unused.

wowchemy/layouts/partials/site_head.html

@@ -101,9 +101,16 @@
     {{- $stylesheets = $stylesheets | append (resources.Get (printf "css/libs/%s.css" . ) ) -}}
   {{- end -}}
   {{ $stylesheets = $stylesheets | resources.Concat "css/vendor-bundle.css" | minify }}
+  {{- if eq (getenv "WC_POST_CSS") "true" -}}
+    {{- $stylesheets = $stylesheets | postCSS -}}
+  {{- end -}}
   {{- if hugo.IsProduction -}}
     {{- $stylesheets = $stylesheets | fingerprint "md5" -}}
   {{- end -}}
+  {{- if eq (getenv "WC_POST_CSS") "true" -}}
+    {{/* PostProcess must be last action in the pipeline */}}
+    {{- $stylesheets = $stylesheets | resources.PostProcess -}}
+  {{- end -}}
   <link rel="stylesheet" href="{{$stylesheets.RelPermalink}}" media="print" onload="this.media='all'">
 
   {{ $scr.Set "vendor_css_filename" "main.min.css" }}