[WIP] Add option to cancel a build

~~this is a rough draft, I'm actively working on it.. so it may throw errors and it may have dirty code.~~

   - Add cancelling and cancelled status for builds
   - Remove pod and update builds to cancelled when a cancel event was triggered for the build
see above, I don't think we need a flag just for the build name, just use args[0] to get the ID of the build. The 'verb' (*cancel-**build***) already containst 'build' so no need for duplication in flags etc.