Don't convert external markdown to .html links (#8)

Author: katniny

links-to-html.lua

@@ -1,4 +1,6 @@
 function Link(el)
-    el.target = string.gsub(el.target, "%.md", ".html")
+    if not el.target:match("^https?://") then
+      el.target = el.target:gsub("%.md$", ".html")
+    end
     return el
   end