Generator output severity labels

[shabbyrobe]

Would it be possible to add labels to the generator output so we can extract them with our Jenkins server?

Something like this:

WARN event.go: SitePlayStart: SiteID: non-local identifier: SiteID
WARN event.go: SiteUnknown: SiteID: non-local identifier: SiteID
INFO event.go: shim: time.Duration -> Int64
INFO event.go: tuple: DevicePlayPrepare

Looks like it'd be super easy to do (happy to send a PR if there's consensus), but just wanted to check if that would be welcome, if there's a downside I'm not seeing, or if there's a better way.