Support load cookies.txt
add global variable cookies_txt

Test case does not enter via script_main.
Without this definition test cases will fail,
although the new feature works correctly.
Merge branch 'add_cookies_merge_commit' of https://github.com/hupili/you-get into hupili-add_cookies_merge_commit
Merge branch 'hupili-add_cookies_merge_commit' into develop