Improve type naming when using pflag

[dlo9]

When using pflag, which is required with viper or cobra, flag types show up like this, which looks very odd:

      --count simpleType[int16]             
      --enabled                 
      --hosts strSliceVar         
      --map strToStrMapVar 
      --timeout duration         (default 0s)

This PR implements Type() string for all custom types so that they show up as normal go types rather than flagfiller's wrapper types, which is used by pflag to resolve the type name

[itzg]

Very nice! Happy to see even more cross integration being possible.

[itzg]

Included in https://github.com/itzg/go-flagsfiller/releases/tag/v1.15.2