gog-download: Use existing cookies for lgogdownloader.

As described in #36 (comment 458372204), this patch changes gog-fetch to use the cookies in ~/.config/lgogdownloader instead of logging in for each download. This avoids problems with captchas which cannot be solved noninteractively, and doesn’t write credentials to the store (access tokens/cookies are still written).

This works-for-me™ but could use some more testing and polishing.