Labelled sub_customer extension as experimental
https://groups.google.com/forum/#!topic/simpleinvoices/IhuRt0y8aRY
Mailing List: https://groups.google.com/forum/#!forum/simpleinvoices
Google Groups Mailing List Link Added
https://groups.google.com/forum/#!forum/simpleinvoices
Merge pull request #69 from apmuthu/master

sub_customer extension labelled as experimental