Allow filtering on complete/incomplete

e.g.:
List all completed tasks
`gask ls :x`
List all incomplete tasks
`gask ls :o`

model.go

@@ -121,6 +121,15 @@ func (a *AppState) getFilterPredicate(filter string) func(todotxt.Task) bool {
 	}
 	filterParts := strings.Split(filter, " ")
 	for _, part := range filterParts {
+		if part == ":x" {
+			predicates = append(predicates, func(t todotxt.Task) bool {
+				return t.Completed
+			})
+		} else if part == ":-x" || part == ":o" {
+			predicates = append(predicates, func(t todotxt.Task) bool {
+				return !t.Completed
+			})
+		}
 		if strings.HasPrefix(part, "@") {
 			predicates = append(predicates, func(t todotxt.Task) bool {
 				for _, v := range t.Contexts {