store output_folder relative to home

[milahu]

if output_folder is a subdirectory of home for example /home/user/rqbit/some.file then store it as ~/rqbit/some.file in .cache/rqbit/session.json

this makes it easier to move home

workaround sed -i -E 's,"/home/user1/,"/home/user2/,g' .cache/rqbit/session.json