fix #59 (#63)

Enable "next word" and "previous word" in many terminals:

* Alt-LeftArrow and Alt-RightArrow in xterm and iTerm2 with default settings,
  where they translate to \x1b\x1b[D and \x1b\x1b[C respectively
* Alt-LeftArrow and Alt-RightArrow in iTerm2 in "Natural text editing mode",
  where they translate to \x1bb and \x1bf
* Alt-LeftArrow and Alt-RightArrow in libvte-based terminals,
  where they translate to \x1b[1;3D and \x1b[1;3C respectively
* Alt-b and Alt-f in xterm, where they translate to \x1bb and \x1bf

terminal.go

@@ -312,12 +312,29 @@ func (t *terminal) ioloop() {
 
 func (t *terminal) consumeANSIEscape(buf *bufio.Reader, ansiBuf *bytes.Buffer) (result readResult, err error) {
 	ansiBuf.Reset()
-	// initial character is either [ or O; if we got something else,
-	// treat the sequence as terminated and don't interpret it
 	initial, _, err := buf.ReadRune()
-	if err != nil || !(initial == '[' || initial == 'O') {
+	if err != nil {
 		return
 	}
+	// we already read one \x1b. this can indicate either the start of an ANSI
+	// escape sequence, or a keychord with Alt (e.g. Alt+f produces `\x1bf` in
+	// a typical xterm).
+	switch initial {
+	case 'f':
+		// Alt-f in xterm, or Option+RightArrow in iTerm2 with "Natural text editing"
+		return readResult{r: MetaForward, ok: true}, nil // Alt-f
+	case 'b':
+		// Alt-b in xterm, or Option+LeftArrow in iTerm2 with "Natural text editing"
+		return readResult{r: MetaBackward, ok: true}, nil // Alt-b
+	case '[', 'O':
+		// this is a real ANSI escape sequence, read the rest of the sequence below:
+	case '\x1b':
+		// Alt plus a real ANSI escape sequence. Handle this specially since
+		// right now the only cases we want to handle are the arrow keys:
+		return consumeAltSequence(buf)
+	default:
+		return // invalid, ignore
+	}
 
 	// data consists of ; and 0-9 , anything else terminates the sequence
 	var type_ rune
@@ -345,9 +362,17 @@ func (t *terminal) consumeANSIEscape(buf *bufio.Reader, ansiBuf *bytes.Buffer) (
 			}
 		}
 	case 'D':
-		r = CharBackward
+		if altModifierEnabled(ansiBuf.Bytes()) {
+			r = MetaBackward
+		} else {
+			r = CharBackward
+		}
 	case 'C':
-		r = CharForward
+		if altModifierEnabled(ansiBuf.Bytes()) {
+			r = MetaForward
+		} else {
+			r = CharForward
+		}
 	case 'A':
 		r = CharPrev
 	case 'B':
@@ -379,6 +404,40 @@ func (t *terminal) consumeANSIEscape(buf *bufio.Reader, ansiBuf *bytes.Buffer) (
 	return // default: no interpretable rune value
 }
 
+func consumeAltSequence(buf *bufio.Reader) (result readResult, err error) {
+	initial, _, err := buf.ReadRune()
+	if err != nil {
+		return
+	}
+	if initial != '[' {
+		return
+	}
+	second, _, err := buf.ReadRune()
+	if err != nil {
+		return
+	}
+	switch second {
+	case 'D':
+		return readResult{r: MetaBackward, ok: true}, nil
+	case 'C':
+		return readResult{r: MetaForward, ok: true}, nil
+	default:
+		return
+	}
+}
+
+func altModifierEnabled(payload []byte) bool {
+	// https://www.xfree86.org/current/ctlseqs.html ; modifier keycodes
+	// go after the semicolon, e.g. Alt-LeftArrow is `\x1b[1;3D` in VTE
+	// terminals, where 3 indicates Alt
+	if semicolonIdx := bytes.IndexByte(payload, ';'); semicolonIdx != -1 {
+		if string(payload[semicolonIdx+1:]) == "3" {
+			return true
+		}
+	}
+	return false
+}
+
 func parseCPRResponse(payload []byte) (cursorPosition, error) {
 	if semicolonIdx := bytes.IndexByte(payload, ';'); semicolonIdx != -1 {
 		if row, err := strconv.Atoi(string(payload[:semicolonIdx])); err == nil {