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).
This does not work for me, same problem.

````
➜ .oh-my-fish git:(kidpixo-master) echo $CDPATH
/Users/bpinto/.marks
➜ .oh-my-fish git:(kidpixo-master) cd functions/
cd: The directory 'functions/' does not exist
````