Add vendor

Specifically because I want to modify a dependency

vendor/github.com/xo/terminfo/stack.go

@@ -0,0 +1,48 @@
+package terminfo
+
+type stack []interface{}
+
+func (s *stack) push(v interface{}) {
+	*s = append(*s, v)
+}
+
+func (s *stack) pop() interface{} {
+	if len(*s) == 0 {
+		return nil
+	}
+	v := (*s)[len(*s)-1]
+	*s = (*s)[:len(*s)-1]
+	return v
+}
+
+func (s *stack) popInt() int {
+	if i, ok := s.pop().(int); ok {
+		return i
+	}
+	return 0
+}
+
+func (s *stack) popBool() bool {
+	if b, ok := s.pop().(bool); ok {
+		return b
+	}
+	return false
+}
+
+func (s *stack) popByte() byte {
+	if b, ok := s.pop().(byte); ok {
+		return b
+	}
+	return 0
+}
+
+func (s *stack) popString() string {
+	if a, ok := s.pop().(string); ok {
+		return a
+	}
+	return ""
+}
+
+func (s *stack) reset() {
+	*s = (*s)[:0]
+}