feed: add support to filter posts

The post filter is kept very simple, it works by checking if a fixed
string exists in the configured field.  This could easily be expanded
allowing for regex or even more complex filters like date-time
comparisons.

hotdog/command/feed.py

@@ -2,6 +2,7 @@ import asyncio
 import logging
 from datetime import datetime, timezone
 from html import escape
+from typing import *
 
 import feeder
 import postillon
@@ -12,12 +13,26 @@ from ..models import Job, Message
 log = logging.getLogger(__name__)
 
 
+def make_filter(config: Mapping[str, str]) -> Callable[[feeder.Post], bool]:
+    def filter(post):
+        return all(
+            text in getattr(post, attrname, "") for attrname, text in config.items()
+        )
+
+    return filter
+
+
 def init(bot):
     bot.on_command("feed", handle)
 
     if "feeder" not in bot.shared:
         feeds = (
-            feeder.Feed(fid, f["url"], title=f["display"])
+            feeder.Feed(
+                fid,
+                f["url"],
+                title=f["display"],
+                filter=make_filter(f.get("filter", {})),
+            )
             for fid, f in bot.config.get("feeder.feeds").items()
         )
         feedstore = feeder.Store(bot.config.get("feeder.storage"))