Adds timestamp flags and global prefix configuration

Prefix is useful for multi-tail scenarios (tailing logs across multiple machines, to help distinguish which is which.)

![selfie-0](http://i.imgur.com/EmaVurf.gif)

Yeah. Sensible default is empty. 

I see what you're saying... As we're moving away from env configs, this is kind of a step in the wrong direction. We can remove the env var configs and leave it up to the project using this lib to explicitly load from env and perform the setup.

OTOH, given it'll be used across our stack and nothing actually bombs (nor does it make testing harder), leaving these env vars as an option is nbd. What say you?