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))
if package can't be located, treat it the same way as disabled

There will be a message "Unable to locate <package-name>" in the log.
Merge pull request #95 from YoungFrog/fix-mult-mode

Eval backquote earlier and support non-`progn' lists.
Merge pull request #104 from alexko/master

if package can't be located, treat it the same way as disabled
Revert "Merge pull request #104 from alexko/master"

This reverts commit 2385ebd291359cf59086608e60a000cc8a92cf03, reversing
changes made to ec16c4f514d71237196acff36c714fb279b67fb1.