ansi: inconsistent handling of tabs

[pachecot]

ansi.StringWidth treats tabs as zero width, but ansi.Harwrap treats tabs a non-zero width of 1.

one would get would unexpected results with a construct such as this :

	s := "foo\tbar"
	n := ansi.StringWidth(s)           // n == 6
	p := ansi.Hardwrap(s, n, true)

	// p == "foo\tba\nr"   
	// not  "foo\tbar" as one might expect 

not sure if this is a bug or intended behavior.

one of the tests for Hardwrap is..

	{"tab", "foo\tbar", 3, "foo\n\tba\nr", true},

[aymanbagabas]

Thank you, @pachecot, for opening this issue. I've opened #238 to patch this one 🙂