introduced .tar.bz2 snapshots

 - reworked write_tar_gzip_archive to handle arbitrary filter as a
   write_compressed_tar_archive
 - reformatted whitespaces in the said function to adhere to common cgit
   standards
 - added wrappers around write_compressed_tar_archive for .tar.gz and .tar.bz2
 - added a hint for vim to use 8 characters shift width by default

Signed-off-by: Michael Krelin <hacker@klever.net>