Add nerdcomment, pdv, muli-cursors, php-namespace

- Replace tcomment with nerdcomment.
- Replace vip (php docblocks) with pdv.
- Add multiple-cursors (like Sublime Text).
- Add php-namespace to auto include `use` statements for selected
  classes.
- Update Readme; links to Plugins, moved to upfront, re-ordered for
  series of setup steps you'll probably have to do.
- Add leader key of , (comma)