Allow configurable log format

[efritz]

Loggers which write to free-form out (standard out, files, etc) should be configurable. Right now there's no timestamp displayed on console output.

It would also be nice if you could adjust which part of the log message is colorized (instead of the entire thing). Go-logging does this by replacing log strings based on patterns. For example:

%{file} %{time} [%{level:8}] %{message}

It would be nice to also specify only part of the message should be colorized (for example the level instead of the entire line). Having the entire line colorized makes it a bit hard to read.