add ac-library to template

Author: atcoder-live

template.cpp

@@ -1,6 +1,8 @@
 #include <bits/stdc++.h>
-#define rep(i,n) for (int i = 0; i < (n); ++i)
 using namespace std;
+#include <atcoder/all>
+using namespace atcoder;
+#define rep(i,n) for (int i = 0; i < (n); ++i)
 using ll = long long;
 using P = pair<int,int>;