Added definition for aggregating individual line items

I added a definition for some syntactic sugar. Examples added to the readme, and fixtures/suite added for test-kitchen.

```ruby
set_limits "nobody" do
  type '-'
  item 'nofile'
  value '4096'
end

set_limits "nobody" do
  type 'hard'
  item 'nproc'
  value '1024'
end
```