-d is relative to the $PWD

This case fails:

    ~% lsrc -d foo -d bar
    /home/mike/.zshrc:/home/mike/foo/zshrc

Because:

    skipping non-existent directory: /home/mike/foo/bar

However, giving the absolute path fixes it:

    ~% lsrc -d $PWD/foo -d $PWD/bar
    /home/mike/.zshrc:/home/mike/foo/zshrc
    /home/mike/.vimrc:/home/mike/bar/vimrc

In this commit we fix this by storing the user's working directory when
they start, and always `cd`ing back before changing dotfile directory.
In this way they are always relative to the current working directory.

Fixes #21.