Add support for Brotli (*.br) compressed files

Brotli is a compression which is mostly used for Web.

This PR is a backport from Engrampa but it also adds a support for *.tar.br files.

But please note that *.br files aren't recognized yet by the file so Nautilus won't show these files as an archive. For some reason it looks like package installer is not working properly so you may need to install brotli package yourself:

sudo apt install brotli
# create a tar.br archive
ar -v -p -cf file.tar.br --use-compress-program=brotli -- file.txt