Use readlink to read symbolic instead cd

Introduced readlink to read symbolic link in ~/.marks/ without need to cd to the link location and back. Thanks to a comment of [seidler2547](https://twitter.com/seidler2547/status/497444546926886912).
Right, but I tend to put only -gx (global in current session and  export  to child processes) in my `fish.conf` to be able to have session with different variables. I never used -U , maybe is a better approach! 

BTW, I removed the 

           set -gx CDPATH . ~ $CDPATH

line from my `fish.conf` and now it is working fine : 

           $ echo $CDPATH
           /Users/damo_ma/.marks . /Users/kidpixo

it contains now $MARKPATH, . and $HOME, that's strange...