[skip ci]

If you do `flag.allowStdin: true` then oclif will read from stdin for
you if the value is `-`, so there's no way to know if the flag value
came from stdin or the user specified a file path (here we need to read
the file).

I don't like having another specific flag just to read from stdin so I
made this a string flag and handle reading from stdin/file in
`flag.parse`.