Print matches while still searching.

Previously we printed the matches at the end.

src/main.cpp

@@ -204,6 +204,14 @@ int main(int argc, char *argv[])
         }
         futurepool.emplace_back(
             std::async(std::launch::async, search_file, filepath));
+
+        if (!matches_all.empty())
+        {
+            output::print_matches(matches_all[0], opts,
+                                  input_files.size() == 1);
+            std::lock_guard<std::mutex> guard(mutex_matches_all);
+            matches_all.erase(matches_all.begin());
+        }
     }
     futures_cleanup(true);