add set num levels api

src/rocksdb_options.rs

@@ -550,6 +550,12 @@ impl Drop for FlushOptions {
             rocksdb_ffi::rocksdb_flushoptions_destroy(self.inner);
         }
     }
+
+    pub fn set_num_levels(&mut self, n: c_int) {
+        unsafe {
+            rocksdb_ffi::rocksdb_options_set_num_levels(self.inner, n);
+        }
+    }
 }
 
 #[cfg(test)]