Feature: Add check mode

[thnee]

Would be useful if it could just verify if something would change, without actually changing the files. So that it can be used as part of a test suite.

See the --check flag in black for comparison.

It could print something like "x number of changes would be made to file zzz", and exit with 0 if there are no changes, and exit with 1 if there are changes.

[f18m]

I'd love to have this feature as well.
Right now I implemented in a mixed bash/C++ process some rules like that:

all:
	@for bashfile in $$(ls *.sh); do \
		cp -f $$bashfile /tmp ; \
		rm -f /tmp/$$bashfile.bak ; \
		beautysh --backup --files /tmp/$$bashfile ; \
		if [ -f /tmp/$$bashfile.bak ]; then \
			echo "Invalid formatting for $$bashfile" ; \
			exit 2 ; \
		else \
			echo "Valid formatting for $$bashfile" ; \
		fi ; \
	done

[lovesegfault]

@thnee, @f18m Has graciously gone ahead and implemented this, I've been overloaded with work lately. Please test it on the master branch!

[f18m]

Sorry @bemeurer but I think that by mistake commit #258330f has removed the --check option previously checked in. My bad.
Do you want me to resubmit the PR for the --check option?

[lovesegfault]

@f18m No worries, please re-submit it 👍