Weather plugin updates

The cache time is now configurable by the user.  If no cache time is
set, the old default of half an hour is still used.

Temperature formatting has been factored out into its own function.

kayleevc/plugins/darksky.py

         "latitude": USER_LATITUDE,
         "longitude": USER_LONGITUDE
     }
+
+Weather information is cached for up to ``cache_max_age`` seconds
+(default 1800, half an hour).  To conserve API calls, the cache is not
+updated until the first time the user requests weather information
+after the cache becomes stale.
 """
 
 import json
             self.options['latitude'],
             self.options['longitude']
         )
-        self._cache_max_age = 1800
+        try:
+            self._cache_max_age = self.options['cache_max_age']
+        except NameError:
+            self._cache_max_age = 1800
 
         self.commands = {
             'whats the temperature': self._temperature,
 
         self.corpus_strings.update(self.commands)
 
+    def _format_temperature(self, temperature, unit='Fahrenheit'):
+        """Format a temperature for the TTS system"""
+        return '{:.1f} degrees {}'.format(temperature, unit)
+
     def _temperature(self):
-        return '{:.1f} degrees Fahrenheit'.format(
-            self.cache['currently']['temperature'])
+        return self._format_temperature(self.cache['currently']['temperature'])
 
     def _todays_low(self):
-        return '{:.1f} degrees Fahrenheit'.format(
+        return self._format_temperature(
             self.cache['daily']['data'][0]['temperatureMin'])
 
     def _todays_high(self):
-        return '{:.1f} degrees Fahrenheit'.format(
+        return self._format_temperature(
             self.cache['daily']['data'][0]['temperatureMax'])
 
     def _tomorrows_high(self):
-        return '{:.1f} degrees Fahrenheit'.format(
+        return self._format_temperature(
             self.cache['daily']['data'][1]['temperatureMax'])
 
     def _relative_humidity(self):