Add caller (file:line number) to non-tracing entries?

Any opinions on this? It's available from `runtime.Caller`. Following the current design, it could be enabled by having a function like `log.IncludeCaller(bool)` and then every entry would include a field `caller` with value `file:line number`). Could also implement this as the default log library handles flags, e.g. `log.SetFlags(log.LstdFlags | log.Lshortfile)`.

I'm happy to create a PR for this if you'd like.