`brcat` should support multiple files as input

`cat` stands for con`cat`enate, 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.

Start by locating the brcat command entry point and reviewing how it currently accepts a single input or concatenated stream. Try the commands in the issue with separate Brotli files and with a pre-concatenated stream; done means multiple file arguments produce the same output as the piped comparison.

Written by the indexing model from the issue text.