Add temp_precision option to darksky plugin

This gives the darksky plugin a temp_precision option, which allows
users to set the precision with which temperatures are read.  It
defaults to 0, not the old hard-coded value of 1, because I decided that
it's not usually useful to hear that much information.

kayleevc/plugins/darksky.py

 (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.
+
+Temperatures are read to the nearest ``temp_precision`` decimal places
+(default 0).
 """
 
 import json
             self._cache_max_age = self.options['cache_max_age']
         except KeyError:
             self._cache_max_age = 1800
+        try:
+            self._temp_precision = self.options['temp_precision']
+        except KeyError:
+            self._temp_precision = 0
 
         self.commands = {
             'whats the temperature': self._temperature,
 
     def _format_temperature(self, temperature, unit='Fahrenheit'):
         """Format a temperature for the TTS system"""
-        return '{:.1f} degrees {}'.format(temperature, unit)
+        return '{:.{prec}f} degrees {}'.format(temperature, unit,
+                prec=self._temp_precision)
 
     def _temperature(self):
         return self._format_temperature(self.cache['currently']['temperature'])