Support ${CAUSE_BUILD_URL} also in addition to ${CAUSE}

Currently there is ${CAUSE} that prints the upstream job that triggered this build. It would be nice to have a the details of the cause available in tokens also, especially the URL. Then the mail can contain a clickable link, which is quite convenient in some case then a build pipeline exists, where one want to get to the "main" job directly.

Having these token available would be really nice:

• ${CAUSE_BUILD_URL}: equivalent of ${BUILD_URL} 
• ${CAUSE_PROJECT_NAME}: equivalent of ${PROJECT_NAME} 
• ${CAUSE_BUILD_NUMBER}: equivalent of ${BUILD_NUMBER}