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.

feeder/models.py

@@ -15,10 +15,15 @@ FeedId = str
 PostId = str
 
 
+def no_filter(x):
+    return True
+
+
 @dataclass
 class Feed:
     id: FeedId
     url: str
+    filter: Callable[["Post"], bool] = no_filter
     title: Optional[str] = None
     posts: List["Post"] = field(default_factory=list)
     etag: Optional[str] = None
@@ -55,13 +60,14 @@ class Feed:
         if "title" in r.feed:
             self.title = r.feed.title
 
-        posts = [Post.from_entry(e) for e in r.entries]
+        posts = [p for e in r.entries if self.filter(p := Post.from_entry(e))]
         for post in posts:
             if post.date is None:
                 post.date = pubdate(r.feed)
         posts.sort(key=lambda e: e.date, reverse=True)
         self.posts = posts
 
+        # Find link to next feed page.
         for link in r.feed.get("links", []):
             if link.get("rel") == "next":
                 self.next_url = link.get("href")