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"
 

tools/whitespace_linter

@@ -0,0 +1,11 @@
+#!/usr/bin/env bash
+
+# git 1.9.0+ allows for exclusions in pathspecs via ':!foo' syntax.
+# However, until git 2.13.0 there must be at least one "inclusive" pathspec, hence the './*'
+trailing_whitespace=$(git grep -E -n -- ' +$' -- './*' ':!*.png')
+
+if [[ -n "$trailing_whitespace" ]]; then
+  printf "ERROR: Trailing whitespaces:\n"
+  awk 'BEGIN {FS=":"} {printf " * %s:%s\n", $1, $2}' <<< "$trailing_whitespace"
+  exit 1
+fi