Only add log.Lmicroseconds if log.Ltime is set
Only add log.Lmicroseconds if log.Ltime is set

Also, move it out of Serve so it's possible to override.

zenazn/goji,w4ngyi/goji,yangou/goji,Invacio/goji,dbmlabs/goji,beni55/goji,unixorn/goji,tchssk/goji,vsco/goji,cognusion/goji,vanackere/slim