Allow the user to choose where to store the urls

[rfc-2549]

In c35ca47d5baa36bea47b40149c86ec92df2b4430 $STORE_LINKS was added, allowing the user to keep track of the files they have uploaded. However, unless you change line 121, the file by defalt is $HOME/.cache/lainsafelinks

I think I should add $LINKS_FILE or something. so line 121 can be like:

open FILE,'>>',$LINKS_FILE or die $!;