Quasiquote

Following up on the idea raised in #50, I have implemented implicit quasiquoting for all non-sexp use-package arguments.

@dudebout's example then becomes
```elisp
(setq my-bind-for-magit '("C-c i" . magit-status))
(use-package magit
  :bind ,my-bind-for-magit)
```

and also allows for @DarwinAwardWinner's:
```elisp
(use-package magit
  :bind ,(get-bindlist-for-package 'magit))
```

and even fancier things which can be seen in the commit message.

Comments welcome.

--

Note that I didn't keep compatibility with @dudebout's auto eval of symbols, since it doesn't require a huge config change, and that code hasn't been out long enough to have really gotten entrenched. Although compatibility could be added easily.