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).
```
Shouldn't we create universal variables if none exists?

if set -q CDPATH
  set -U CDPATH $MARKPATH $CDPATH
else
  set -U CDPATH . ~ $MARKPATH
end
```