Expose Value implementations to use with FlagSet.AddFlag

[mattysweeps]

It would be useful when dynamically adding Flags to a FlagSet to be allowed to reuse the underlying primitive implementations of a Value.

The boolean implementation for reference: https://github.com/spf13/pflag/blob/master/bool.go#L13

This makes it difficult to use the Flag struct directly because I have to create something identical to boolValue.

[quite]

Was about to file this issue. Yes, this would be very useful when, at a higher level, abstracting creation of a cli command (for example using Cobra), and having a []*pflag.Flag that should each be added to the command.

I think that exporting (capitalizing) all new*Value functions. Possibly, exporting each different type that these functions return should/needs to be done as well.

[quite]

I've provided a PR in #432 . It exports the new*Value function for every type, and all the corresponding types.