fix: make callout detection case-insensitive (closes #171)

Jacky Zhao

2022-08-05 5c76d8dad9d993010c4e57c6de4e90911a3cd11a

 layouts/_default/single.html

@@ -10,7 +10,7 @@
    <article>
      {{if .Title}}<h1>{{ .Title }}</h1>{{end}}
      <p class="meta">
          Last updated {{if ne .Date .Lastmod}}{{ .Lastmod.Format "January 2, 2006" }}{{else}}Unknown{{end}}. 
          Last updated {{ partial "date-fmt.html" .}} 
          {{ partial "github.html" . }}
      </p>
      <ul class="tags">