Add `--force` option to `make check-formatting`

This causes the check to proceed even where there are unstaged changes in the
working tree.

Signed-off-by: Michael Richters <gedankenexperimenter@gmail.com>

Makefile

@@ -113,6 +113,7 @@ check-formatting:
 	bin/format-code.py \
 		--exclude-dir 'testing/googletest' \
 		--exclude-file 'generated-testcase.cpp' \
+		--force \
 		--check \
 		--verbose \
 		src plugins examples testing