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.

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