Add ability to specify custom build number in Pipelines (i.e. the version)

[Brett Mathe]

For our team we use semver and automatically generate our version number using commit history and tags.

It would be great if the Pipeline # that shows on the left could be replaced with a variable from the pipeline itself.

For example, looking at a number like #15 means nothing to me but if I saw 1.2.1-rc.23 I would know what artifacts to look for in my deployment software.  

[Chelsea Engert]

This is a great idea