Support including the ThreadId in the output

[ssokolow]

I have a project where I currently manually include the output of thread::current().id() at the beginning of every log message (prefixing the error string and padded to ensure the error string messages align) and it'd be very nice to just have an option I can turn on to move the responsibility for that into stderrlog.

...that or just supporting a "set custom formatter callback" sledgehammer.