Move seen config example in place so it loads

In general, plugins in Cardinal should load without creating a config
file unless intervention is necessary (e.g. an API key is required to
use the plugin.)

The `seen` plugin required you to copy `config.json.example` to
`config.json` even though there are no secrets in the config file.

plugins/seen/plugin.py

@@ -21,6 +21,9 @@
 
 class SeenPlugin:
     def __init__(self, cardinal, config):
+        if config is None:
+            config = {}
+
         self.cardinal = cardinal
         self.ignored_channels = config.get('ignored_channels', [])