Selector. __str__()  method

I often use functions that implicitly call ```str()``` upon their arguments
and having to prepend an extract() in the composition seems too noisy to
me. I would expect ```__str__()``` to return the extracted text of the selected
node all along. This doesn't and shouldn't concern SelectorList at all.
I don't see a reason for a __collection__ to have such a method.

I saw many PRs discussing methods for single node extraction,
joined string results etc. I do not mean to replace the functionality
suggested by them.

A note on unicode: I don't understand why this works with unicode text,
I thought ```__str__``` can only return an encoded bytearray. Please comment.