Add rubocop binstub that rspects file patterns

Best of both worlds!

(Because you can't override the default rubocop includes)

The binstub basically, lets me safely `rubocop test/foo_test.rb`
instead of `bundle exec rubocop test/foo_test.rb`

```bash
  # ~/.profile
  # https://twitter.com/tpope/status/165631968996900865
  # tl;dr `mkdir .git/safe` to add `bin` to path, e.g. `bin/rails`
  PATH=".git/safe/../../bin:$PATH"
```

.rubocop.yml

@@ -1,10 +1,13 @@
 AllCops:
   TargetRubyVersion: 2.1
   Exclude:
-    - config/initializers/forbidden_yaml.rb
     - !ruby/regexp /(vendor|bundle|bin|db|tmp)\/.*/
   DisplayCopNames: true
   DisplayStyleGuide: true
+  # https://github.com/bbatsov/rubocop/blob/master/manual/caching.md
+  # https://github.com/bbatsov/rubocop/blob/e8680418b351491e111a18cf5b453fc07a3c5239/config/default.yml#L60-L77
+  UseCache: true
+  CacheRootDirectory: tmp
 
 Rails:
   Enabled: true