Add 'at' time trigger (#493)

## Description:


**Related issue (if applicable):** fixes <link to issue>

**Pull request in [esphome-docs](https://github.com/esphome/esphome-docs) with documentation (if applicable):** esphome/esphome-docs#<esphome-docs PR number goes here>
**Pull request in [esphome-core](https://github.com/esphome/esphome-core) with C++ framework changes (if applicable):** esphome/esphome-core#<esphome-core PR number goes here>

## Checklist:
  - [ ] The code change is tested and works locally.
  - [ ] Tests have been added to verify that the new code works (under `tests/` folder).

If user exposed functionality or configuration variables are added/changed:
  - [ ] Documentation added/updated in [esphomedocs](https://github.com/OttoWinter/esphomedocs).

esphome/config_validation.py

@@ -2,6 +2,7 @@
 """Helpers for config validation using voluptuous."""
 from __future__ import print_function
 
+from datetime import datetime
 import logging
 import os
 import re
@@ -13,7 +14,7 @@ from esphome import core
 from esphome.const import CONF_AVAILABILITY, CONF_COMMAND_TOPIC, CONF_DISCOVERY, CONF_ID, \
     CONF_INTERNAL, CONF_NAME, CONF_PAYLOAD_AVAILABLE, CONF_PAYLOAD_NOT_AVAILABLE, CONF_PLATFORM, \
     CONF_RETAIN, CONF_SETUP_PRIORITY, CONF_STATE_TOPIC, CONF_TOPIC, ESP_PLATFORM_ESP32, \
-    ESP_PLATFORM_ESP8266
+    ESP_PLATFORM_ESP8266, CONF_HOUR, CONF_MINUTE, CONF_SECOND
 from esphome.core import CORE, HexInt, IPAddress, Lambda, TimePeriod, TimePeriodMicroseconds, \
     TimePeriodMilliseconds, TimePeriodSeconds, TimePeriodMinutes
 from esphome.py_compat import integer_types, string_types, text_type, IS_PY2
@@ -391,6 +392,23 @@ positive_not_null_time_period = vol.All(time_period,
                                         vol.Range(min=TimePeriod(), min_included=False))
 
 
+def time_of_day(value):
+    value = string(value)
+    try:
+        date = datetime.strptime(value, '%H:%M:%S')
+    except ValueError as err:
+        try:
+            date = datetime.strptime(value, '%H:%M:%S %p')
+        except ValueError:
+            raise vol.Invalid("Invalid time of day: {}".format(err))
+
+    return {
+        CONF_HOUR: date.hour,
+        CONF_MINUTE: date.minute,
+        CONF_SECOND: date.second,
+    }
+
+
 def mac_address(value):
     value = string_strict(value)
     parts = value.split(':')