Make colourizing the timestamp optional

[bpeetz]

As outlined in #51, colouring the whole message makes it hard to read, as readers cannot
predict what would be written there (whereas INFO, ERROR, etc. are rather obvious).
This is also a problem with a colourized timestamp.

Additionally, this would be in line with the colourization employed by other log implementations (e.g. env_log).

[cardoe]

Sure. Want to submit a PR?