hex_taball to be able to unpack list of files

[RudolfMan]

I would like to be able to extract particular file from hex_tarball.. (needed for this hexpm/diff#53)
Introduce second argument that might be a list of filenames and pass it down to hex_erl_tar along with the list of options in tuple {files, ListOfFiles} or an atom all_files (so that we could default unpack/2 to it)

unpack(Tarball, Output) ->
  unpack(Tarball, all_files, Output, hex_core:default_config()).

@ericmj do you think it's possible to move recently introduced last config param from hex_tarball:unpack/3 to hex_param:unpack/4 while master is not released yet?
Sorry for this late PR (should have done earlier)

[ericmj]

It looks like we do not need this for hexpm/diff#53. Closing this for now, we can always reopen later.