Add a simple whitespace linter

The goimports tool does not check for trailing whitespaces in multiline
strings. This commit adds a simple linter to address this deficiency.

This change will also have the benefit of providing light linting of any
non-go files.

Change-Id: I253dd7c4895f476fa3a6f0c4fccf7c9cada16366

Makefile

@@ -80,6 +80,7 @@ fmt: lint
 lint: tidy
 lint: $(LINTER)
 	@echo "Performing linting step..."
+	@./tools/whitespace_linter
 	@./$(LINTER) run --config $(LINTER_CONFIG)
 	@echo "Linting completed successfully"