Bind OutdentSelection to ShiftTab (Backtab)

I was under the impression that tcell couldn't detect shift tab
key presses, but this was incorrect. It turns out that tcell just
calls it backtab. This commit makes backtab the default binding
for OutdentSelection.

See #203

cmd/micro/bindings.go

@@ -370,6 +370,7 @@ func DefaultBindings() map[string]string {
 		"Alt-Backspace":  "DeleteWordLeft",
 		"Alt-Backspace2": "DeleteWordLeft",
 		"Tab":            "IndentSelection,InsertTab",
+		"Backtab":        "OutdentSelection",
 		"CtrlO":          "OpenFile",
 		"CtrlS":          "Save",
 		"CtrlF":          "Find",