Add --check option to pgindent

Not sold on the name, but --check is a combination of --silent-diff and
--show-diff. I envision --check mostly being used in CI environments.
I recently came across a situation where this behavior would have been
useful. Without --check, you're left to capture the output of
--show-diff and exit 2 if the output isn't empty by yourself.

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Tristan Partin
Neon (https://neon.tech)