New -n/--no-header argument: don't write global archive header

Allows using shell redirection to append files to existing archive. Eg.:

  $ ptar c foo/ba* > foo.ptar
  $ cat foo.ptar | ptar t
  foo/bar
  foo/baz
  $ ptar -n c foo/buz >> foo.ptar
  $ cat foo.ptar | ptar t
  foo/bar
  foo/baz
  foo/buz
Wording, grammar
Merge pull request #3 from czarkoff/master

New -n/--no-header argument: don't write global archive header