Added StringSliceVarMult

[cdoern]

the purpose of this new flag declaration is so the user can put the data of the flag
in multiple locations without having to manually assign it in their application.
This is useful with entities that inherit many properties from a parent.

Example usage:

testFlagName := "test"
var combined []*[]string
combined = append(combined,  &createOpts.Parent)
combined = append(combined,  &createOpts.Child)
flagSet.StringSliceVarMult(combined, testFlagName, []string{}, "fill multiple entities at once")

[CLAassistant]

All committers have signed the CLA.

[cdoern]

closing due to staleness