Enable the spaced-comment ESLint rule

Please see http://eslint.org/docs/rules/spaced-comment.

Note that the exceptions added for `line` comments are intended to still allow use of the old preprocessor without linting errors.
Also, I took the opportunity to improve the grammar slightly (w.r.t. capitalization and punctuation) for comments touched in the patch.

.eslintrc

@@ -105,5 +105,14 @@
     "space-in-parens": ["error", "never"],
     "space-infix-ops": ["error", { "int32Hint": false }],
     "space-unary-ops": ["error", { "words": true, "nonwords": false, }],
+    "spaced-comment": ["error", "always", {
+      "line": {
+        "exceptions": ["//", "#else", "#endif"],
+        "markers": ["#if", "#elif", "#include", "#expand", "#error"],
+      },
+      "block": {
+        "balanced": true,
+      }
+    }],
   },
 }