Add docinfo API in extension

Resolves #1162
I think it would be good to add the pos attribute to the DSL so it can be configured as:

```ruby
SampleDocinfoProcessor < Extensions::DocinfoProcessor
  use_dsl
  at_position :footer
  def process document, attributes
    ...
  end
end
```

wdyt? do you like the name `at_position`, or does something else feel more correct to you?