Add urlloader example.

Demonstrate loading files from URL.

examples/urlloader.janet

@@ -0,0 +1,28 @@
+# An example of using Janet's extensible module system
+# to import files from URL. To try this, run `janet -l examples/urlloader.janet`
+# from the repl, and then:
+#
+# (import https://raw.githubusercontent.com/janet-lang/janet/master/examples/colors.janet :as c)
+#
+# This will import a file using curl. You can then try
+#
+# (print (c/color :green "Hello!"))
+#
+# This is a bit of a toy example (it just shells out to curl), but it is very
+# powerful and will work well in many cases.
+
+(defn- load-url
+  [url args]
+  (def f (file/popen (string "curl " url)))
+  (def res (dofile f :source url ;args))
+  (try (file/close f) ([err] nil))
+  res)
+
+(defn- check-http-url
+  [path]
+  (or (string/has-prefix? "http://" path)
+      (string/has-prefix? "https://" path)))
+
+# Add the module loader and path tuple to right places
+(array/push module/paths ["HTTP" :janet-http check-http-url identity])
+(put module/loaders :janet-http load-url)