Add hide default value in help/usage

[nateph]

Currently if you add a default value through NoOptDefVal or DefValue, it gets printed in the help/usage message.

fs.StringVarP(&cs.Password, "password", "p", "default", "the password for a remote user supplied by -u or --user.")
fs.Lookup("password").NoOptDefVal = "default"

results in

-p, --password string[="default"]   the password for a remote user supplied by -u or --user. (default "default")

This field in the Flag type would omit the [="default"] and (default "default") additions.

[CLAassistant]

All committers have signed the CLA.

[AdamSLevy]

#205

[erezpaz125]

Hi, can this PR be merged? This issue is relevant for a project at my workplace and it would help alot. Thank you!