Allow to enable only certain speechhandlers by configuration (#176)

Thanks for pointing out about the master/naomi-dev change with the readme.

I just tested and both this:
`speechhandlers: ["life","clock","Shutdown","joke","unclear"]`
and this:
```
speechhandlers:
  life
  clock
  Shutdown
  joke
  unclear
```
work. The second looks odd and I'm not sure it's proper yaml but it works.

naomi/application.py

@@ -259,7 +259,9 @@ def __init__(
                     exc_info=(
                         self._logger.getEffectiveLevel() == logging.DEBUG))
             else:
-                self.brain.add_plugin(plugin)
+                if 'speechhandlers' not in self.config or info.name in self.config['speechhandlers']:
+                  self._logger.info('Activate speechhandler plugin %s', info.name)
+                  self.brain.add_plugin(plugin)
 
         if len(self.brain.get_plugins()) == 0:
             msg = 'No plugins for handling speech found!'