keep date as a type of data in DataFrame-js

[feiyu-london]

it seems to me that the data types in dataframe-js are two kinds, i.e. number and string.

Though I agree that many date related functions can be supplemented by functions under javascript, but from a data dealing perspective date type is a quite important and useful feature.

Converting everything to milisecond can solve the problem in terms of process, but it can be quite difficult for people to realise what they are dealing with without transforming them back to proper date formate.

If the dataframe can recognize various date format as date rather than as string, it can be very powerful and convenient for users.

Hope this makes sense.