[loc] Add extractor (Closes #3188)

Added extractor of loc.gov, which closes #3188. I am not an experienced programmer, so I am sure I did a bunch of mistakes, but the extractor works (for me at least).

[LibraryOfCongress] don't use video_id for _search_regex()

[LibraryOfCongress] Improvements

youtube_dl/extractor/libraryofcongress.py

@@ -0,0 +1,65 @@
+# coding: utf-8
+from __future__ import unicode_literals
+
+from .common import InfoExtractor
+
+from ..utils import determine_ext
+
+
+class LibraryOfCongressIE(InfoExtractor):
+    _VALID_URL = r'https?://(?:www\.)?loc\.gov/item/(?P<id>[0-9]+)'
+    _TESTS = [{
+    'url': 'http://loc.gov/item/90716351/',
+        'info_dict': {
+            'id': '90716351',
+            'ext': 'mp4',
+            'title': 'Pa\'s trip to Mars /'
+        },
+        'params': {
+            # m3u8 download
+            'skip_download': True,
+        }
+    }, {
+        'url': 'https://www.loc.gov/item/97516576/',
+        'only_matching': True,
+    }]
+
+    def _real_extract(self, url):
+        video_id = self._match_id(url)
+        webpage = self._download_webpage(url, video_id)
+
+        self.report_extraction(video_id)
+        json_id = self._search_regex('media-player-([0-9A-Z]{32})', webpage, 'json id')
+
+        data = self._parse_json(self._download_webpage(
+            'https://media.loc.gov/services/v1/media?id=%s' % json_id,
+            video_id), video_id)
+        data = data['mediaObject']
+
+        media_url = data['derivatives'][0]['derivativeUrl']
+        media_url = media_url.replace('rtmp', 'https')
+
+        is_video = data['mediaType'].lower() == 'v'
+        if not determine_ext(media_url) in ('mp4', 'mp3'):
+            media_url += '.mp4' if is_video else '.mp3'
+
+        if media_url.index('vod/mp4:') > -1:
+            media_url = media_url.replace('vod/mp4:', 'hls-vod/media/') + '.m3u8'
+        elif url.index('vod/mp3:') > -1:
+            media_url = media_url.replace('vod/mp3:', '')
+
+        formats = []
+        if determine_ext(media_url) == 'm3u8':
+            formats = self._extract_m3u8_formats(media_url, video_id, ext='mp4')
+        elif determine_ext(media_url) is 'mp3':
+            formats.append({
+                'url': media_url,
+                'ext': 'mp3',
+            })
+
+        return {
+            'id': video_id,
+            'thumbnail': self._og_search_thumbnail(webpage),
+            'title': self._og_search_title(webpage),
+            'formats': formats,
+        }