Adding new property web.defaultOrigin.

The web.defaultOrigin property allows you to specify what origin name is used within the "Empty Repository" page that is displayed after creation of a repository.

Reason for PR: people within my organization had difficulty understanding the use of "gitblit" instead of "origin" and using the standard "origin" was better for our purposes of explaining Git to users.

NOTE - I didn't know what to fill in as the SINCE value, so I left that as ??? Other than that it was tested locally and found to be working.