Add example for *repl-prompt*

Author: mpwillson

examples/*repl-prompt*.janet

@@ -0,0 +1,9 @@
+(var ncmd 0)
+
+(defn my-prompt
+  `Takes a single argument p, the parser, and returns a prompt string.`
+  [p]
+  (++ ncmd)
+  (string/format "[%d]> " ncmd))
+
+(setdyn :repl-prompt my-prompt)