Consider `Else` overloads that return a `Maybe<T>`

[j2jensen]

This could allow chaining fallbacks:

var value = GetValueFromCacheMaybe().Else(() => GetValueFromDatabaseMaybe()).Else(DefaultValue);

[j2jensen]

A brief proof of concept shows that this extension method doesn't seem to conflict with the other Else method. The compiler is still able to infer the return type when it's used.

        public static Maybe<T> Else<T>(this Maybe<T> src, Func<Maybe<T>> elseSelector)
        {
            return src.HasValue ? src : elseSelector();
        }