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"
```

lib/tasks/rubocop.rake

@@ -0,0 +1,53 @@
+begin
+  require 'rubocop'
+  require 'rubocop/rake_task'
+rescue LoadError # rubocop:disable Lint/HandleExceptions
+else
+  require 'rbconfig'
+  # https://github.com/bundler/bundler/blob/1b3eb2465a/lib/bundler/constants.rb#L2
+  windows_platforms = /(msdos|mswin|djgpp|mingw)/
+  if RbConfig::CONFIG['host_os'] =~ windows_platforms
+    desc 'No-op rubocop on Windows-- unsupported platform'
+    task :rubocop do
+      puts 'Skipping rubocop on Windows'
+    end
+  elsif defined?(::Rubinius)
+    desc 'No-op rubocop to avoid rbx segfault'
+    task :rubocop do
+      puts 'Skipping rubocop on rbx due to segfault'
+      puts 'https://github.com/rubinius/rubinius/issues/3499'
+    end
+  else
+    Rake::Task[:rubocop].clear if Rake::Task.task_defined?(:rubocop)
+    patterns = [
+      'Gemfile',
+      'Rakefile',
+      'lib/**/*.{rb,rake}',
+      'config/**/*.rb',
+      'app/**/*.rb',
+      'test/**/*.rb'
+    ]
+    desc 'Execute rubocop'
+    RuboCop::RakeTask.new(:rubocop) do |task|
+      task.options = ['--rails', '--display-cop-names', '--display-style-guide']
+      task.formatters = ['progress']
+      task.patterns = patterns
+      task.fail_on_error = true
+    end
+
+    namespace :rubocop do
+      desc 'Auto-gen rubocop config'
+      task :auto_gen_config do
+        options = ['--auto-gen-config'].concat patterns
+        require 'benchmark'
+        result = 0
+        cli = RuboCop::CLI.new
+        time = Benchmark.realtime do
+          result = cli.run(options)
+        end
+        puts "Finished in #{time} seconds" if cli.options[:debug]
+        abort('RuboCop failed!') if result.nonzero?
+      end
+    end
+  end
+end