Adds two engines : Youtube with or without API

The API needs an API_KEY
The NOAPI doesn't have the published dates.

searx/engines/youtube_noapi.py

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+# Youtube (Videos)
+#
+# @website     https://www.youtube.com/
+# @provide-api yes (https://developers.google.com/apis-explorer/#p/youtube/v3/youtube.search.list)
+#
+# @using-api   no
+# @results     HTML
+# @stable      no
+# @parse       url, title, content, publishedDate, thumbnail, embedded
+
+from urllib import quote_plus
+from lxml import html
+from searx.engines.xpath import extract_text
+
+# engine dependent config
+categories = ['videos', 'music']
+paging = True
+language_support = False
+
+# search-url
+base_url = 'https://www.youtube.com/results'
+search_url = base_url + '?search_query={query}&page={page}'
+
+embedded_url = '<iframe width="540" height="304" ' +\
+    'data-src="//www.youtube-nocookie.com/embed/{videoid}" ' +\
+    'frameborder="0" allowfullscreen></iframe>'
+
+base_youtube_url = 'https://www.youtube.com/watch?v='
+
+# specific xpath variables
+results_xpath = "//ol/li/div[contains(@class, 'yt-lockup yt-lockup-tile yt-lockup-video vve-check')]"
+url_xpath = './/h3/a/@href'
+title_xpath = './/div[@class="yt-lockup-content"]/h3/a'
+content_xpath = './/div[@class="yt-lockup-content"]/div[@class="yt-lockup-description yt-ui-ellipsis yt-ui-ellipsis-2"]'
+
+
+# do search-request
+def request(query, params):
+    params['url'] = search_url.format(query=quote_plus(query),
+                                      page=params['pageno'])
+
+    return params
+
+
+# get response from search-request
+def response(resp):
+    results = []
+
+    dom = html.fromstring(resp.text)
+
+    # parse results
+    for result in dom.xpath(results_xpath):
+        videoid = result.xpath('@data-context-item-id')[0]
+
+        url = base_youtube_url + videoid
+        thumbnail = 'https://i.ytimg.com/vi/' + videoid + '/hqdefault.jpg'
+
+        title = extract_text(result.xpath(title_xpath)[0])
+        content = extract_text(result.xpath(content_xpath)[0])
+
+        embedded = embedded_url.format(videoid=videoid)
+
+        # append result
+        results.append({'url': url,
+                        'title': title,
+                        'content': content,
+                        'template': 'videos.html',
+                        'embedded': embedded,
+                        'thumbnail': thumbnail})
+
+    # return results
+    return results