Add linkbot contrib module

Linkbot listens for http:// and https:// links, and attempts to get
their titles.

This also adds the "tesla" dependency to mix.exs

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

lib/contrib/linkbot.ex

@@ -0,0 +1,38 @@
+defmodule Omnibot.Contrib.Linkbot do
+  use Omnibot.Module
+  require Logger
+
+  defmodule Client do
+    use Tesla
+
+    plug Tesla.Middleware.Headers, [{"user-agent", "Tesla/Omnibot"}]
+    plug Tesla.Middleware.FollowRedirects, max_redirects: 10
+    plug Tesla.Middleware.Compression, format: "gzip"
+
+    @title_regex ~r"<title>(?<title>.+)</title>"i
+
+    def get_title(url) do
+      if should_get?(url) do
+        resp = get!(url)
+        %{"title" => title} = Regex.named_captures(@title_regex, resp.body)
+        title
+      end
+    end
+
+    defp should_get?(url) do
+      resp = head!(url)
+      Tesla.get_header(resp, "content-type")
+      |> String.downcase()
+      |> String.contains?(["html", "text"])
+    end
+  end
+
+  @url_regex ~r"\bhttps?://[^\s]+"
+
+  @impl true
+  def on_channel_msg(irc, channel, _nick, line) do
+    Regex.scan(@url_regex, line)
+    |> Enum.map(fn url -> Client.get_title(url) end)
+    |> Enum.each(fn title -> Irc.send_to(irc, channel, title) end)
+  end
+end