Resolve "Add pop function"

Closes #6 (closed)

stack.c

+#include "stack.h"
+
+void stack_init(stack *s){
+    s->top = -1;
+    s->capacity = DEFAULT_CAPACITY;
+    s->data = malloc(sizeof(int) * DEFAULT_CAPACITY);
+}
+
+bool stack_is_empty(stack s){
+	return s.top == -1;
+}
+
+void stack_peek(stack s, int *value){
+    if (!stack_is_empty(s)) {
+        *value = s.data[s.top];
+    }
+}
+
+// depile
+void stack_pop(stack *s, int *val){
+	if (stack_is_empty(*s)){
+		return;
+	}
+	*val = s->data[s->top];
+	s->top -= 1;
+	printf("pop : %d, top : %d\n", s->data[s->top],s->top);
+}