IssueTracker

I would like to include a build time version number into my application. A common way to do this is via the `go -ldflags "-X main.Version 1.5"` flag. I would like goapp to support this.

This version is described in a few places:
https://golang.org/cmd/link/
https://github.com/golang/go/wiki/GcToolchainTricks#including-build-information-in-the-executable
https://www.atatus.com/blog/golang-auto-build-versioning/
https://blog.cloudflare.com/setting-go-variables-at-compile-time/

  * What is the desired behaviour of the feature? (Be specific!)

$ goapp deploy -application myapp-1234 -version 1234 -ldflags "-X main.Date=2017-02-16" app.yaml

This would build the binary with the variable main.Date set to '2017-02-16'.

* If relevant, why are current approaches or workarounds insufficient?

Currently I work around this by using a Makefile, sed and a versions.go file. This is doable, but means I have to do something different for my Appengine builds.

* If relevant, what new use cases will this feature will enable?

This will enable embedding better build time information into the application, such as version and date.

thanks.