Support go template in generator

The generator package now can

• generate methods to meet the strange log.LoggableStruct interface
• port support of go template from Ayi, previously logging level in log package is generated by Ayi gotmpl, now its part of gommon generate and defined in gommon.yml without passing data in command line flag
• gommon generate is using go's built in flag package, instead of cobra, go's flag package is not very handy