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 is breaking `cd` for some reason.

I can no longer use `cd` to go inside one any folder:
````
➜ .oh-my-fish git:(kidpixo-master) ✗ ls
LICENSE         README.markdown custom          functions       oh-my-fish.fish plugins         templates       themes          tools
➜ .oh-my-fish git:(kidpixo-master) ✗ cd custom/
cd: The directory 'custom/' does not exist
````