rcup -f and -i

Add the `-f` and `-i` options to rcup(1). `-f` will overwrite any file
it doesn't recognize; `-i` will prompt each time, and is the default.

For example:

    rm ~/.zshrc
    touch ~/.zshrc
    rcup -f # overwrite that .zshrc with the symlink
    rcup -i # prompt whether to overwrite that .zshrc