tabwriter: Introduce a new flag StripEscape to control
if tabwriter.Escape chars should be stripped or passed
through unchanged.

go/printer: Don't modify tabwriter.Escape'd text. This
involved a new implementation of the internal trimmer
object.

Does not affect formatting of any existing code under
$GOROOT/src and $GOROOT/misc.

Fixes #1030.

R=rsc
CC=golang-dev
https://golang.org/cl/1943045