New property steam in ep-lh-0020.md

[romanrostislavovich]

Some engines can return output in stderr communication channel.

I suggest to add an extra property to pipeline item: stream: it will specify what to pass to the next pipeline item (stdout or stderr of the previous pipe).

Please, add steam property to ep-lh-0020.md in pipeline item

Follow property can next values:

Value	Default
stdout	true
stderr	false