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).
Interesting, I don't have this variable defined. I've fish installed from git.

````
➜ src if set -q PATH; echo 'Exists'; else; echo 'Does not exist'; end
Exists
➜ src if set -q CDPATH; echo 'Exists'; else; echo 'Does not exist'; end
Does not exist
````