short flags for --yaml-input and --yaml-output

[imarko]

Add -y and -Y as short flags for --yaml-input and --yaml-output.

[imarko]

I've been using these in my local copy so that I can gojq -yY for easy YAML processing but always assumed that it wasn't in upstream because of (future) flag compatibility caution. But maybe that's not the case and I don't see a previous issue or PR so I figure it's worth a try.

[itchyny]

This could be useful for users switching these flags, but give me more time to decide. In my environment, I have a simple shell alias to enable these options and that's working well. Also, I'm afraid users might be confusing about the short flag capitals; especially for being the opposite against the raw string flags.

[itchyny]

I can't see any benefit at the risk of short flag collision in the future. Also these flags may confuse the users due to the reason in my previous comment.