core: Basic proof-of-concept working with support for zsh right now

Not all commands are implemented, but I was able to do the following
successfully:

    $ cd
    $ dnav add home
    New alias created home -> /home/posborne
    $ cd Projects/dnav
    $ dnav add dnav
    New alias created dnav -> /home/posborne/Projects/dnav
    $ dnav cd home
    $ pwd
    /home/posborne
    $ dnav cd dnav
    $ pwd
    /home/posborne/Projects/dnav