User-configurable time and level

[ernesto-jimenez]

It might be good to allow configurations to:

1. Disable time or tweak the format
2. Change the name of the field for severity or disable it entirely. e.g.: Google Cloud Logging expects a severity rather than a level field.

[raphael]

Both of these are already possible via custom formatters, a bit on the fence as to whether it's worth adding options on top? Also, the time information is important to enable the buffering feature (the time the log entry is created is not necessarily the same as the time the log entry is written) so I'm not sure there should be a mechanism to disable it and let people shoot themselves in the foot - could be convinced otherwise though.