Better integration of Option<T> with string formatting

[atifaziz]

This is a follow-on issue from #17.

If #19 is merged then the issue of formatting still remains; that is, the following:

var d = new DateTime(2010, 12, 31).Some();
var s = string.Format(new CultureInfo("de-DE"), "{0:D}", d);
Console.WriteLine(s);

will print:

31/12/2010 00:00:00

This is better than printing Some(31/12/2010 00:00:00) but not the expected value of Freitag, 31. Dezember 2010.

Option<T> could be made to integrate better with string formatting by making it implement IFormattable and then format the value (when Some) using the format and IFormatProvider passed to IFormattable.ToString().