Update USE_CASES.md
Update USE_CASES.md

Added an example of how to use CustomArgs as requested https://github.com/sendgrid/sendgrid-go/issues/108
sendgrid/sendgrid-go,karmakaze/sendgrid-go,karmakaze/sendgrid-go,VividCortex/sendgrid-go,sendgrid/sendgrid-go,VividCortex/sendgrid-go