Handle fmt.Stringer types that are passed as nil pointers (#13)

Co-authored-by: Will Owens <ghthor@gmail.com>

Fix interface conversion when Kind() == string but v.(string) panics (#…

…12)

Handle Nil Logging Fields (#10)

This commit avoids panics when there are nil, logging fields which are accepted by Logrus. Also updates the repo to use modules.