Issue #153: Add support for SameSite cookie flag

Add support for SameSite cookie flag in PHP >= 7.3.0. I split this out as a separate if statement to keep existing functionality in 7.2.x as similar as possible. When dropping support for 7.2.x, the idea would be to turn the method into just what's in side the version-comparing `if`.

src/Cookie.php

@@ -140,6 +140,7 @@ protected static function getConfig(string $stored): SymmetricConfig
      * @param string $domain (defaults to NULL)
      * @param bool $secure   (defaults to TRUE)
      * @param bool $httpOnly (defaults to TRUE)
+     * @param string $samesite (defaults to ''; PHP >= 7.3.0)
      * @return bool
      *
      * @throws InvalidDigestLength
@@ -156,16 +157,34 @@ public function store(
         string $path = '/',
         string $domain = '',
         bool $secure = true,
-        bool $httpOnly = true
+        bool $httpOnly = true,
+        string $sameSite = ''
     ): bool {
+        $val = Crypto::encrypt(
+            new HiddenString(
+                (string) \json_encode($value)
+            ),
+            $this->key
+        );
+        if (\version_compare(PHP_VERSION, '7.3.0') >= 0) {
+            $options = [
+                'expires' => (int) $expire,
+                'path' => (string) $path,
+                'domain' => (string) $domain,
+                'secure' => (bool) $secure,
+                'httponly' => (bool) $httpOnly,
+            ];
+            if ($sameSite !== '') {
+                $options['samesite'] = (string) $sameSite;
+            }
+            return \setcookie(
+                $name,
+                $val,
+                $options);
+        }
         return \setcookie(
             $name,
-            Crypto::encrypt(
-                new HiddenString(
-                    (string) \json_encode($value)
-                ),
-                $this->key
-            ),
+            $val,
             (int) $expire,
             (string) $path,
             (string) $domain,