scripts/merge-pr.py: Manually map Github username to email address

You can get pretty much any old email address out of Github API, so
let's just map the emails manually.

.github.json

@@ -0,0 +1,6 @@
+{
+  "penberg": {
+    "name": "Pekka Enberg",
+    "email": "penberg@ki.fi"
+  }
+}