Support IsSet() method on flags

[nikhiljindal]

To distinguish when the flag got default value because user didnt set any or user explicitly set the flag to same value as the default value.

[n10v]

You can use Changed func to check it:

if cmd.Flags().Changed("someFlag") {
    // user set the flag
}

[nikhiljindal]

Thanks! That worked.

[xieyuschen]

Shall we also provide a function like:

func (f *FlagSet) GetChangedList() []string 

So we could register a handler map to process the corresponding flags.

[johntdyer]

@xieyuschen that would be nice