Download stories when --load-cookies is used

[ekalin]

Fixes #2259.

Stories could only be downloaded if --login was used. I changed this to either --login or --load-cookies, which also logs the user in.

Otherwise the user needed to run Instaloader first just to import the cookies, and a second time (using the saved session) to actually download stories.

I've also changed error messages and the documentation in several places to refer to the process of logging in rather than to --login directly, but if you think that is too confusing, feel free to discard that commit.

• Is it just a proof of concept? No
• Is the documentation updated (if appropriate)? Yes
• Do you consider it ready to be merged or is it a draft? Ready
• Can we help you at some point? There's nothing I need help for, but I'm open to suggestions.

[aandergr]

Thanks!

I also like the changes to the documentation and the wordings in output messages. To me it seems they make the program easier to understand and more user-friendly.