Make plugin base classes abstract

Now each plugin base class (PluginBase and Handler) has an abstract
method for the main functionality it provides, forcing plugin authors to
implement those methods before their plugin can be successfully used.

kayleevc/plugins/__init__.py

 # Copyright 2015-2017 Clayton G. Hobbs
 # Portions Copyright 2013 Jezra
 
+"""Builtin Kaylee plugins and base classes"""
 
-class PluginBase():
+from abc import ABC, abstractmethod
+
+
+class PluginBase(ABC):
     """Base class for Kaylee plugins
 
     Each Kaylee plugin module must define a subclass of this class, named
         self.options = config.plugins[name]
         self.corpus_strings = set()
 
+    @abstractmethod
     def get_handler(self, text):
         """Return a handler for the given text
 
         the command is recognized, but for some reason would not perform any
         action currently, e.g. "pause music" when no music player is running.
         """
-        return None
 
 
-class Handler:
+class Handler(ABC):
     """Base class for Kaylee plugin handlers
 
     Plugins should subclass this for their own command handlers.
         """
         self.confidence = confidence
 
+    @abstractmethod
     def __call__(self, tts):
         """Handle the command