Add automatic cover image support (#303)

Sean

23.34.2020 5001b28387cc9bc7ae0e592856d90ac53a8328e3

 layouts/post/summary-with-image.html

@@ -1,12 +1,13 @@
<article class="bb b--black-10">
  <a class="db pv4 ph3 ph0-l no-underline dark-gray dim" href="{{ .Permalink }}">
    <div class="flex flex-column flex-row-ns">
      {{ if .Params.featured_image }}
      {{ $featured_image := partial "func/GetFeaturedImage.html" . }}
      {{ if $featured_image }}
        <div class="pr3-ns mb4 mb0-ns w-100 w-40-ns">
          <img src="{{ .Params.featured_image }}" class="db" alt="image from {{ .Title }}">
          <img src="{{ $featured_image }}" class="db" alt="image from {{ .Title }}">
        </div>
      {{ end }}
      <div class="w-100{{ if .Params.featured_image }} w-60-ns pl3-ns{{ end }}">
      <div class="w-100{{ if $featured_image }} w-60-ns pl3-ns{{ end }}">
        <h1 class="f3 fw1 athelas mt0 lh-title">{{ .Title }}</h1>
        <div class="f6 f5-l lh-copy nested-copy-line-height">
          {{ .Summary }}