Add support for customizing the prefix format

[santigl]

glog uses a fixed format for the prefix that is attached to log messages, which seems to follow what Google's internal tools expect (#229 (comment)).

However, other users of glog could need to integrate with tools that expect other formats, or would like to make the information more human-friendly (for example, by showing the full names of log levels instead of their initials), or might want to display the date in other timezones. They might also want to replace another logging library that has its own format and start using glog.

It would be great if glog allowed users of the library to specify their own prefix format. The current format would remain the default, and if the implementation is gated behind a compile-time flag, there wouldn't be any penalty for users that do not need the functionality.

[drigz]

@sergiud Any thoughts about this and the linked PR?

[elftech]

I'm looking forward to this too.