Work towards #56: Add permalink to every pattern

This makes cleaner URLs thanks to jekyll

double-dispatch/index.md

@@ -2,6 +2,7 @@
 layout: pattern
 title: Double Dispatch
 folder: double-dispatch
+permalink: /patterns/double-dispatch/
 categories: pattern_cat
 tags: pattern_tag
 ---
@@ -17,4 +18,4 @@ behavior based on receiver and parameter types.
 
 **Real world examples:** 
 
-* [ObjectOutputStream](https://docs.oracle.com/javase/8/docs/api/java/io/ObjectOutputStream.html)
+* [ObjectOutputStream](https://docs.oracle.com/javase/8/docs/api/java/io/ObjectOutputStream.html)