Add a script to run 'go vet'

@filbranden 
Works for me...  

```
$ go vet ./...
pkg/api/types.go:276: struct field tag `json:"namespace",omitempty" yaml:"namespace,omitempty"` not compatible with reflect.StructTag.Get
exit status 1
pkg/api/v1beta1/types.go:266: struct field tag `json:"namespace",omitempty" yaml:"namespace,omitempty"` not compatible with reflect.StructTag.Get
exit status 1
[snip]
```