Add ability to prefix with current time.

I'd love to be able to set this in my local config file.

Use-cases include debugging a program using this where we want a server to respond within a certain time.

Another use-case is to write a integration test where we want services to respond within a time-frame. In the latter case, a delta from the previous output might be even more practiacal.

Suggested syntax: Similar to `docker log -t` which is:
`2016-12-21T01:38:57.215005634Z `