Add datetime type and timezone support

[item4]

I can use date type but it can not save time info.

Please add datetime type

Known Issue:

• timezone?

[mitsuhiko]

Yep. That's missing, largely because it needs a story for timezones.

[item4]

@mitsuhiko Do you have any plan about datetime for official type? or I want to make plugin for my blog.

[niijv]

Datetime is planned as an official type as far as I know.

[mitsuhiko]

I will merge this, just need docs and everything else too which is why it's not done yet.

[mitsuhiko]

This one is actually in now.