Using an interface allows people to use their favourite flag system
with viper without being restricted to the semantics of pflag or the
standard library.
This change introduce two new functions `BindFlagValues` and
`BindFlagValue` that behave like `BindFlags` and `BindFlag` but using
the new interface as values.
This change also introduces two internal structures to transform
`*pflag.FlagSet` and `*pflag.Flag` into the new interface. This way,
viper keeps working as expected for people that are currently using the
pflag package without breaking backwards compatibility.
Signed-off-by: David Calavera <david.calavera@gmail.com>
