Example implementation of date issue

I was thinking of ways to implement date as we discussed also making is generic. 
And it seems to me that the best way is to have pandoc.date and pandoc.date.return methods.
In that case we can use it for [POSIXct, POSIXc, Date](https://github.com/RomanTsegelskyi/pander/blob/6b12eccd26ac03b6b1c37d552995a1d650731a42/R/S3.R#L387-L397) and also for [zoo](https://github.com/RomanTsegelskyi/pander/blob/6b12eccd26ac03b6b1c37d552995a1d650731a42/R/S3.R#L517-L518) and in future other classes.