use-package: use defun as lisp-indent-function

When `use-package' is called with only one keyword it is useful to
write:

(use-package foo :init
  (progn
    ... long lines ...))

instead of

(use-package foo
  :init (progn
  	  ... *too* long lines ...))

or

(use-package foo
  :init
  (progn
    ... long lines ...))

Even when there are multiple keywords or when one never wants to format
the calls to `use-package' as in the first example the use of `defun'
does not really pose a problem.
Merge pull request #69 from tarsius/defun

use-package: use defun as lisp-indent-function
allow :mode and :interpreter to accept a string

Fixes #72.
Merge pull request #73 from aspiers/short-mode

allow :mode and :interpreter to accept a string