Fix support for piping STDIN

[mhssmnn]

Since the change to Open3 STDIN is not sent to prettier, so formatting produces an empty output.

This sets the :stdin_data option to pass Open3 the STDIN.

The previous use of system seems to have implicitly used the parent STDIN.

[kddnewton]

Nice fix, thanks!

[molawson]

@kddnewton I seem to be having the opposite problem with this now. I and a coworker noticed separately that calling bundle exec rbprettier --write . (really, any CLI options behaved the same) would seem to hang indefinitely, and we wouldn't see anything until we hit <Enter> a second time.

It looks like with this change, Open3.capture3 is waiting for input from STDIN to close before showing any output. The command appears to be running in the background but no output is shown.

When I run this locally, removing the stdin_data arg makes the command run without extra input, and it prints the lists of files it has processed. Is there something I'm missing here?