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: some 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.