Add script.wait action (#778)

Fixes https://github.com/esphome/feature-requests/issues/416, fixes https://github.com/esphome/issues/issues/572

tests/test3.yaml

@@ -383,6 +383,8 @@ text_sensor:
       - lambda: !lambda |-
           ESP_LOGD("main", "The state is %s=%s", x.c_str(), id(version_sensor).state.c_str());
       - script.execute: my_script
+      - script.wait: my_script
+      - script.stop: my_script
       - homeassistant.service:
           service: notify.html5
           data: