Add a new shortcode for referencing lines in a GitHub repo

Including Chapel as a submodule would add 1GB to clones.

layouts/shortcodes/githubsnippet.html

@@ -0,0 +1,10 @@
+{{- $repo := .Get 0 -}}
+{{- $commit := .Get 1 -}}
+{{- $file := .Get 2 -}}
+{{- $lang := .Get 3 -}}
+{{- $line := .Get 4 -}}
+{{- $lines := split (trim .Inner "\n") "\n" -}}
+
+{{- $url := printf "https://github.com/%s/blob/%s/%s#L%d" $repo $commit $file $line -}}
+{{- partial "linerangestr.html" (dict "scratch" .Scratch "from" 0 "to" $line) -}}
+{{- partial "highlightgroup.html" (dict "url" $url "path" $file "comment" (.Scratch.Get "comment") "code" (trim .Inner "\n") "language" $lang) -}}