.vimrc: enable 'delimitMate' plugin

From delimitMate README:

  This plug-in provides automatic closing of quotes, parenthesis,
  brackets, etc., besides some other related features that should make
  your time in insert mode a little bit easier, like syntax awareness
  (will not insert the closing delimiter in comments and other
  configurable regions), <CR> and <Space> expansions (off by default), and
  some more.

  Most of the features can be modified or disabled permanently, using
  global variables, or on a FileType basis, using :autocmd. With a couple
  of exceptions and limitations, these features don't break undo, redo or
  history.

I've tried this plugin once before and ditched it. I want to try it
again. And if if is annyoing, at least try to configure it this time
instead of expecting perfect "defaults".