Add rcdn

This program will remove all your rc files that are symlinked. This can
be further controlled by `-d` and `-t`.

For example, you can feel comfortable trying new rc files because you
can quickly remove them again with `-d`.

    rcup -d thoughtbot-dotfiles
    rcdn -d thoughtbot-dotfiles

Likewise, when you're done with Python just drop it:

    rcdn -t python