1) Add EscapeCodes to the terminal so that applications don't wire
   them in.
2) Add a callback for auto-complete
3) Fix an issue with input lines longer than the width of the
   terminal.
4) Have Write() not stomp the current line. It now erases the current
   input, writes the output and reprints the prompt and partial input.
5) Support prompting without local echo in Terminal.
6) Add GetSize to report the size of terminal.

R=bradfitz
CC=golang-dev
https://golang.org/cl/5479043