DefaultLoader/FlagLoader is unusable

[smthpickboy]

A common case for production is you need to use a command line parameter to specify the path of config file, then read and parse it. But that parameter option is apparently not included in FlagLoader's FlagSet, which will result in

flag provided but not defined

error.

Need to remove it from DefaultLoader.

[markx]

If that's not supported out of box, I think you can just create a separate file loader and load that config file again into the config struct.