add OrComposeDecodeHookFunc function

[julnicolas]

The aim is to be able to compose existing or custom DecodeHookFuncs conditionally.

Currently it is possible to chain calls thanks to ComposeDecodeHookFunc but execution is aborted in case an error is found. Therefore, ComposeDecodeHookFunc can be seen as an AND logical operation.

OrComposeDecodeHookFunc on the other hand, executes until a function returns a nil error or all functions returned a non-nil-error. It can be seen as a complimentary OR logical operation.

One practical use case is to be able to parse dates with different supported layouts :

        data := FormData{}
	config := &mapstructure.DecoderConfig{
		DecodeHook: mapstructure.ComposeDecodeHookFunc(
			OrComposeDecodeHookFunc(
				// Converts input.type === "month"
				mapstructure.StringToTimeHookFunc("2006-01"),
				// Converts input.type === "date"
				mapstructure.StringToTimeHookFunc("2006-01-02"),
				// Converts input.type === "month" from other layout
				mapstructure.StringToTimeHookFunc("2006/01"),
				// Converts input.type === "date" from other layout
				mapstructure.StringToTimeHookFunc("2006/01/02"),
			),
                        // Other DecodeHookFuncs
		),
		WeaklyTypedInput: true,
		Result:           &data,
	}

[mitchellh]

Beautiful thanks. I think helping composition of hooks is a great thing to add.