Pad log level so output is more aligned

[killercup]

This makes the example print this:

TRACE: 2017-12-07T13:44:04Z: default: some trace log
DEBUG: 2017-12-07T13:44:04Z: default: some debug log
 INFO: 2017-12-07T13:44:04Z: default: some information log
 WARN: 2017-12-07T13:44:04Z: default: some warning log
ERROR: 2017-12-07T13:44:04Z: default: some error log

[euclio]

I personally prefer the padding on the right side. That's what pretty-env-logger does.

[KodrAus]

Thanks @killercup! I think aligning the level to the colon is totally fine. I personally agree with @euclio but can't think of a reason why it would be universally better to align the other way.