add feed formats

lib/feed_reader/formats/atom.ex

@@ -0,0 +1,7 @@
+defmodule FeedReader.Formats.Atom do
+  @moduledoc false
+
+  def matches_prefix?(prefix), do: Regex.match?(~r/<feed\b/is, prefix)
+
+  def id, do: :atom
+end

lib/feed_reader/formats/rdf.ex

@@ -0,0 +1,7 @@
+defmodule FeedReader.Formats.RDF do
+  @moduledoc false
+
+  def matches_prefix?(prefix), do: Regex.match?(~r/<rdf:rdf\b/is, prefix)
+
+  def id, do: :rdf
+end

lib/feed_reader/formats/rss.ex

@@ -0,0 +1,9 @@
+defmodule FeedReader.Formats.RSS do
+  @moduledoc false
+
+  def matches_prefix?(prefix) do
+    Regex.match?(~r/<rss\b[^>]*\bversion\s*=\s*(["'])2\.0\1/is, prefix)
+  end
+
+  def id, do: :rss
+end