Add SetFlags to customize the format for the messages - #18

These flags define which text to prefix to each log entry generated by the Logger.

Bits are or'ed together to control what's printed. There is no control over the order they appear (the order listed here) or the format they present (as described in the comments). The prefix is followed by a colon only when Llongfile or Lshortfile is specified. For example:

log.SetFlags(LstdFlags)
// -> 2009/01/23 01:23:23 message

log. SetFlags(Ldate | Ltime)
// -> 2009/01/23 01:23:23 message

log.SetFlags(Ldate | Ltime | Lmicroseconds | Llongfile)
// -> 2009/01/23 01:23:23.123123 /a/b/c/d.go:23: message

More info: https://golang.org/pkg/log/#pkg-constants