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).
I found the problem : even if specified in the documentation

    "the fish configuration defines `CDPATH` to be a universal variable with the values `.` and `~`."

 seems that setting CDPATH delete its previous state, even if I am simply adding a new element. 
That means the  CDPATH will contain only $MARKPATH and not  `.` and `~`, breaking autocompletion in current directory and from your HOME.

A workaround is to add 

     set -gx CDPATH . ~ $CDPATH

to your `fish.conf` of change the declaration in `jump.load` to

   set -gx CDPATH . ~ $MARKPATH $CDPATH