datatable.prettyprint.char could use ellipsis (…) instead of three dots

[tdhock]

this is a silly trivial example, but

> options(datatable.prettyprint.char=2, datatable.print.class=FALSE)
> data.table(x="foo")
       x
1: fo...
> options(datatable.prettyprint.char=3, datatable.print.class=FALSE)
> data.table(x="foo")
     x
1: foo

The first output above is actually wider than the second. (5 characters instead of 3, surprising, because it is supposed to abbreviate)

could we change to below output? (still three characters)

     x
1: fo…

[tdhock]

related to #5425