Support for custom gitUrlPattern or custom repository types

It would be nice if one could provide a custom `_gitUrlPattern`, to generate custom URLs for "View source" and "Improve this Doc"

This would enable us to support "View source" for other repositories then VSO or Github hosted ones.

This would for example enable us to use a special pattern when building a pull request, so that the resulting "view source" would refer the pull request view for a file, instead of the repository view of the file. So a reviewer could look at the generated API documentation, and if he finds something worth commenting, he could press the "View source" button and be taken to the pull-request (instead of beeing taken to something like refs/pull/ repository view)

At the same time you could also use this to support other repository types then Git, like Subversion, P4 or TFVC.