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

this is a silly trivial example, but ```r > 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… ```