Add include-code shortcode (#502)

Author: jarrodmillman

doc/content/example.py

@@ -0,0 +1,2 @@
+def foo(x):
+    return x**2

layouts/shortcodes/include-code.html

@@ -0,0 +1,10 @@
+{{/*
+
+doc: Include a file with syntax highlighting.
+
+{{< include-code "example.py" "python" >}}
+
+*/}}
+
+{{ $filename := (path.Join .Page.File.Dir (.Get 0)) }}
+{{- highlight (readFile $filename) (.Get 1) -}}