Add tar_extra_args attribute to unarchive module

This patch allows to run unarchive module with some tar specific
options I'm often missing a lot. e.g.:

  ---
  - name: Unpack some archive with stupid internal structure
    unarchive:
      src=path/to/archive
      dest=/path/to/unpacked/archive
      tar_extra_args='--strip-components=1'

This is risky: A typo in tar_extra_args might break the unarchive
module without Ansible noticing about it.