Eval backquote earlier and support non-`progn' lists.

* use-package.el (use-package-plist-get): add optional args: `eval-backquote'
  and `no-progn' to control how arguments are retrieved.
  (use-package-plist-get-value): remove this function
  (use-package): replace calls to old function to modified function.

Fixes issue #94.

Rationale :
- use-package-plist-get-value was just another layer for no good reason,
  and IMO its name was totally unclear.
- we now eval-as-backquote earlier, allowing constructs like:
(let ((my-list-of-commands-in-foo '(foo1 foo2)))
  (use-package foo :commands ,@my-list-of-commands-in-foo))