add DefaultFunc for default values that need to be generated

[steverusso]

Probably would be something like:

func (in InputInfo) DefaultFunc(fn func() (string, error)) InputInfo {
	in.DefaultFunc = fn
	return in
}

fn would be called when parsing kicks off to get the default value and feed it into the InputInfo's parser just as StrDefault currently works. Some thoughts:

• Does this need to be mutually exclusive with the current default? I think so.
• Should this be cached? Probably not. However, if so, maybe it can just be cached in StrDefault if the above point holds, which I think it does.

[steverusso]

Current thoughts:

• It should be mutually exclusive with the current default. It should be a builder and validation error to have both.
• It should not be cached. The library consumer can do caching if they'd like to.