VIM: use the system clipboard as the default register

link: http://vimcasts.org/episodes/accessing-the-system-clipboard-from-vim/

quote:

    Tell Vim to use the `quotestar` ("+) register for all yank,
    delete, change, and put operations that have no register
    explicitly specified. In effect, the system clipboard becomes
    the default register. Beware that with this setting, the x
    command will write a single character to the system clipboard,
    overwriting whatever was in there previously.