bots: Add Youtube Bot.

This sends the first video from youtube search results as a reply.

contrib_bots/bots/youtube/readme.md

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+# Youtube bot
+
+Youtube bot is a Zulip bot that can fetch first video from youtube
+search results for a specified term. To use youtube bot you can simply
+call it with `@mention-bot` followed by a command. Like this:
+
+```
+@mention-bot <search term>
+```
+
+![example usage](assets/screen.png)

contrib_bots/bots/youtube/youtube.py

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+# See readme.md for instructions on running this bot.
+import requests
+from bs4 import BeautifulSoup
+
+class YoutubeHandler(object):
+    def usage(self):
+        return '''
+            This bot will return the first Youtube search result for the give query.
+            '''
+
+    def handle_message(self, message, client, state_handler):
+        help_content = '''
+            To use the, Youtube Bot send `@mention-bot search terms`
+            Example:
+            @mention-bot funny cats
+            '''.strip()
+        if message['content'] == '':
+            client.send_reply(message, help_content)
+        else:
+            text_to_search = message['content']
+            url = "https://www.youtube.com/results?search_query=" + text_to_search
+            r = requests.get(url)
+            soup = BeautifulSoup(r.text, 'lxml')
+            video_id = soup.find(attrs={'class': 'yt-uix-tile-link'})
+            try:
+                link = 'https://www.youtube.com' + video_id['href']
+                client.send_reply(message, link)
+            except TypeError:
+                client.send_reply(message, 'No video found for specified search terms')
+
+handler_class = YoutubeHandler