added 2 more parameter variables - ghprbSourceBranch and ghprbPullLink

"ghprbPullLink" - This is the link to Pull-Request on GitHub. So that, the Jenkins Job can "link-back" / "reverse-link" to the Pull-Request that was captured by ghprb in the first place

"ghprbSourceBranch" - This is the name of the source branch for which the Pull-Request was created (this is a nice to have value since it could contain information like Ticket Number which is essential for integration with Tracking Systems (like JIRA, RALLY, BUGZILLA etc…)

Example: PullRequest for "Master" <-- "Bug1234-nirav-fix-issue-abcd" this allows to update the ticket with ID "Bug1234" stating that a PullRequest was opened for this ticket