Initial date option

[jarmo]

Add support for specifying initial date instead of today via options.

[MathiasWP]

Is it a good idea to use Infinity? new Date(Infinity) resolves to an invalid date.

[jarmo]

@MathiasWP yes, giving Infinity or -Infinity to Date constructor would cause invalid date, but it will never happen with this code. Infinity and -Infinity are just used so that Math.min and Math.max would work as needed.

If you think that invalid date might happen then please give me an example of all the values of options.date, options.min and options.max causing this problem.