eslint: Add missing space-before-keywords rule

All examples in README.md seem to agree on avoiding missing whitespaces
before keywords such as:

    if (cond) {
    }else {       // no space before 'else'
    }

    try {
    }catch (e) {  // no space before 'catch'
    }

This patchs adds the `space-before-keywords` rule as an error (as is
`space-after-keywords` already).

packages/eslint-config-airbnb/rules/style.js

@@ -88,8 +88,10 @@ module.exports = {
     'semi': [2, 'always'],
     // sort variables within the same declaration block
     'sort-vars': 0,
+    // require a space before certain keywords
+    'space-before-keywords': [2, 'always'],
     // require a space after certain keywords
-    'space-after-keywords': 2,
+    'space-after-keywords': [2, 'always'],
     // require or disallow space before blocks
     'space-before-blocks': 2,
     // require or disallow space before function opening parenthesis