Ensure featureimage is present before calling resources.GetRemote

Also added ability to set author image as external url and updated
relevant documentation in configuration

layouts/partials/article-link/simple.html

@@ -40,8 +40,10 @@
     {{- $images := $.Resources.ByType "image" -}}
     {{- $featured := $images.GetMatch "*feature*" -}}
     {{- if not $featured }}{{ $featured = $images.GetMatch "{*cover*,*thumbnail*}" }}{{ end -}}
+    {{ if .Params.featureimage }}
     {{- $url:= .Params.featureimage -}}
     {{- if not $featured }}{{ $featured = resources.GetRemote $url }}{{ end -}}
+    {{ end }}
     {{- if not $featured }}{{ with .Site.Params.defaultFeaturedImage }}{{ $featured = resources.Get . }}{{ end }}{{ end -}}
     {{ if .Params.hideFeatureImage }}{{ $featured = false }}{{ end }}
     {{- with $featured -}}