Config propogation from gqlserver to graphql-go

[abhif22]

Through this PR I intend to introduce the following changes:

1. Added method EnablePanicLogging which propogate the config from gqlserver app
2. Create a custom logger which has a field corresponding to config from gqlserver. This field is set in above method to true. When set to true the logs will contain detailed info on query and variables over and above the stack trace from panic otherwise logs will contain just the stack trace.
3. CustomLogger implements interface Logger and overrided method LogPanic

[bmishra]

You're logging either way, may rename the toggle to VerbosePanicLog

msg := fmt.Sprintf("graphql: panic occurred: %v\n%s", value, buf)
if l.VerbosePanicLog {
 msg = fmt.Sprintf("%s\n\n%s", msg, buf)
}

log.Printf(msg)