Make the PluginConfig class heritate the poezio Config

We can use the nice and safe get() method

src/plugin.py

@@ -1,9 +1,10 @@
 import os
 from configparser import ConfigParser
+import config
 import inspect
 import traceback
 
-class PluginConfig(ConfigParser):
+class PluginConfig(config.Config):
     def __init__(self, filename):
         ConfigParser.__init__(self)
         self.__config_file__ = filename
@@ -23,6 +24,7 @@ class PluginConfig(ConfigParser):
         except IOError:
             return False
 
+
 class SafetyMetaclass(type):
     # A hack
     core = None