Prettier printing from el-get-make-recipes

* el-get.el (el-get-write-recipe): print a newline after each :prop val
  recipe pair, and then pretty indent the whole thing. Note that
  standard Emacs pretty-printing doesn't put any newlines in lists
  except around sublists.
Optimize el-get-make-recipes

* el-get.el (el-get-make-recipes): use el-get-read-all-recipes-names
  which uses the file names instead of actually reading the recipe files
  themselves.