`brcat` should support multiple files as input

cat stands for concatenate, its main feature is combining multiple files into single output stream. brcat should do the same, brcat -- file1.br file2.br file3.br is expected to do the same (and hopefully slightly more efficient) as cat -- file1.br file2.br file3.br | brcat. Right now it works with pre-concatenated brotli-compressed streams (using the implicit --concatenated option) internally, but doesn't accept multiple files. This would also align the behaviour with zcat etc.