Added function to set date format

[IvanProdaiko94]

I would like to have a way to define the date format from my own code.

[alexwestside]

This looks like what I needed! Without that I must to write custom UnmarshalJSON for correct date converting.

[jxsl13]

DateFormat is a public global variable that can be set even without that function.

[IvanProdaiko94]

@jxsl13 it was constant before

[jxsl13]

my bad