Allow juice to run with stdin and stdout if no filenames passed

I'd like to be able to use juice to pipe from stdin and send to stdout, for example when using a filter in vim. I know I can do `juice /dev/stdin /dev/stdout`, but it would be more elegant to just execute juice without arguments, or at least support `-` for stdin/stdout.

No files or tests are named. Start by locating the CLI entry point and its handling of missing arguments, then compare the existing /dev/stdin and /dev/stdout behavior with the requested no-argument or '-' forms. Done means juice can read stdin and write stdout when invoked as a filter, with coverage for the supported invocation.

Written by the indexing model from the issue text.