Add support for date to the IDL language

[jryannel]

[jacky309]

The mechanism described at #83 could be used to enable the use of date type.

[jryannel]

Either data can be added as

@date
string today

or

date today

Date would be always date-time. Or simple

string today

And the user needs to know this is an ISO date/time format.

As it looks like there is more benefits in keeping the type format compatible with JSON types.