Allow override of date format (#291)

Lars Kruse

01.45.2021 b0bf3ccca1a431feb7b788b2ec1210aa903bed97

 layouts/post/summary.html

@@ -1,7 +1,7 @@
  <div class="mb3 pa4 mid-gray overflow-hidden">
    {{ if .Date }}
      <div class="f6">
        {{ .Date.Format "January 2, 2006" }}
        {{ .Date.Format (default "January 2, 2006" .Site.Params.date_format) }}
      </div>
    {{ end }}
    <h1 class="f3 near-black">