Enable tslint with basic rules

Add tslint as a plugin to tsc, so that they will show up in vscode or
whatever editor people use.  I only enabled two basic rules that I think
are very useful, prefer-const and no-var-keyword.  Violations of these
rules were fixed with "tslint -p . --fix".  There was one unused
variable (in the isExpandable) function that I removed by hand.

tsconfig.json

@@ -7,10 +7,15 @@
 			"es6"
 		],
 		"sourceMap": true,
-		"rootDir": "."
+		"rootDir": ".",
+		"plugins": [
+			{
+				"name": "tslint-language-service"
+			}
+		]
 	},
 	"exclude": [
 		"node_modules",
 		".vscode-test"
 	]
-}
+}