tools: enable unicode-bom ESLint rule

Disallow UTF-8 Byte Order Marks in the source tree.
That Node can read these files is still tested
by `test/sequential/test-module-loading.js`.

.eslintrc.yaml

@@ -113,6 +113,7 @@ rules:
   space-in-parens: [2, never]
   space-infix-ops: 2
   space-unary-ops: 2
+  unicode-bom: 2
 
   # ECMAScript 6
   # http://eslint.org/docs/rules/#ecmascript-6