Add backward-delete-char/eof action (#1891)

'backward-delete-char/eof' will either abort if query is
empty or delete one character backwards.

src/terminal.go

@@ -187,6 +187,7 @@ const (
 	actAcceptNonEmpty
 	actBackwardChar
 	actBackwardDeleteChar
+	actBackwardDeleteCharEOF
 	actBackwardWord
 	actCancel
 	actClearScreen
@@ -1846,6 +1847,13 @@ func (t *Terminal) Loop() {
 					t.input = []rune{}
 					t.cx = 0
 				}
+			case actBackwardDeleteCharEOF:
+				if len(t.input) == 0 {
+					req(reqQuit)
+				} else if t.cx > 0 {
+					t.input = append(t.input[:t.cx-1], t.input[t.cx:]...)
+					t.cx--
+				}
 			case actForwardChar:
 				if t.cx < len(t.input) {
 					t.cx++