New rule: Require spaces before keywords

This is a new rule, but this style has always been required by
standard. Now we can actually enforce it with this new eslint rule!

Author: feross

eslintrc.json

@@ -114,6 +114,7 @@
     "space-after-keywords": [2, "always"],
     "space-before-blocks": [2, "always"],
     "space-before-function-paren": [2, "always"],
+    "space-before-keywords": [2, "always"],
     "space-in-parens": [2, "never"],
     "space-infix-ops": 2,
     "space-return-throw-case": 2,