refactor Plugin objects to plugins directory

This patch moves all of the plugins out of plugin.py
into their own separate plugins/<plugin>.py file.  This
makes it easier to maintain each plugin.

NOTE:  You will have to update your ~/.config/aprsd/aprsd.yml

to change the python location path for each plugin enabled.

For example:
OLD:
  enabled_plugins:
   - aprsd.plugin.EmailPlugin

TO NEW
  enabled_plugins:
   - aprsd.plugins.email.EmailPlugin

aprsd/plugin.py

@@ -65,6 +65,7 @@ class APRSDPluginBase(metaclass=abc.ABCMeta):
     @abc.abstractmethod
     def command(self, fromcall, message, ack):
         """This is the command that runs when the regex matches.
+
         To reply with a message over the air, return a string
         to send.
         """