more file types;

--- a/src/Tools/VSCode/extension/package.json	Wed Jan 04 21:20:37 2017 +0100
+++ b/src/Tools/VSCode/extension/package.json	Wed Jan 04 21:26:56 2017 +0100
@@ -32,7 +32,7 @@
             {
                 "id": "isabelle-ml",
                 "aliases": ["Isabelle/ML"],
-                "extensions": [".ML"],
+                "extensions": [".ML", ".sml", ".sig"],
                 "configuration": "./isabelle-ml-language.json"
             }