add options to support cross build

.github/CONTRIBUTING.md

@@ -12,7 +12,7 @@ grunt karma:dev
 
 ### Run tests for backend assets before commit
 ```
-test -z "$(gofmt -s -l . | grep -v vendor/src/ | tee /dev/stderr)"
+test -z "$(gofmt -s -l . | grep -v -E 'vendor/(github.com|golang.org|gopkg.in)' | tee /dev/stderr)"
 ```
 
 ### Run tests for frontend assets before commit