Make `fresh edit` follow symlinks

Closes #73.
Use Perl `abs_path` instead of `readlink` to work with relative paths

`realpath` function from @jasoncodes.

https://github.com/jasoncodes/dotfiles/blob/2ce67436206bac617e24a074b60e5d362ef90872/shell/functions/realpath.sh
Add test for using `fresh edit` to resolve relative links