bibfmt: revise flags

[mseri]

To avoid having hacks in some linter configurations, files are explicitly passed with a -f, --files flag and only if this is not present we read from stdin.

In the meantime I added the --force flag.

We should drop --file, make -f (if anything) and alias for --force and simply read all arguments as files. Maybe we can add --stdin or use - as argument to read from stdin.

When doing this we should harmonize the arguments to bibdedup. They both grew up as personal hacks, but since other users have been picking them up, it would make sense also for their benefits to keep it consistent.

The README needs to be updated accordingly.