Add option for very accurate dirty flag

Set the `fastdirty` option flag to off if you really want accurate
reporting on whether the buffer is modified. This is more resource
intensive but it can be useful for people who don't mind.

Closes #787
Closes #467

cmd/micro/view.go

@@ -199,7 +199,7 @@ func (v *View) ScrollDown(n int) {
 // If there are unsaved changes, the user will be asked if the view can be closed
 // causing them to lose the unsaved changes
 func (v *View) CanClose() bool {
-	if v.Type == vtDefault && v.Buf.IsModified {
+	if v.Type == vtDefault && v.Buf.Modified() {
 		var choice bool
 		var canceled bool
 		if v.Buf.Settings["autosave"].(bool) {