Support ansi coloring

[remkop]

This looks nice: https://github.com/backuity/clist

[remkop]

Ansi codes: https://gist.github.com/dainkaplan/4651352
Enabling ansi on Windows (powershell) : https://gist.github.com/chubin/22a239783c67456eb0de
ANSICON works on Windows 7: https://web.liferay.com/ja/web/igor.spasic/blog/-/blogs/enable-ansi-colors-in-windows-command-prompt
General tips: https://github.com/rspec/rspec/wiki/Getting-colored-output-working-on-Windows

[remkop]

AnsiText:
  String plain;
  int offset;
  int length;
  List<Integer> indices;
  List<String > asciEscapes;

substring(int from, int to) {
  assert to >= from && to <= plain.length();
  Text copy = clone();
  copy.offset = this.offset + from;
  copy.length = to - from;
  return copy;
}