Coherent name for AUTOEXTRACT_USER

[ivanprado]

Current names:

scrapy-autoextract -> AUTOEXTRACT_USER
scrapinghub-autoextract -> SCRAPINGHUB_AUTOEXTRACT_KEY

It would be nice to be coherent around the different libraries and the API itself. The API seems to use key https://doc.scrapinghub.com/autoextract.html. So probably better to use KEY instead of USER. And then there is a question regarding if using SCRAPINGHUB_AUTOEXTRACT or just AUTOEXTRACT and even if this could be different per library.

[victor-torres]

Thank you for creating this issue, @ivanprado.

My personal suggestion is to rename from AUTOEXTRACT_USER to AUTOEXTRACT_KEY.