Youtube-search-requests

- fixed safe search is not working
- added channel search (NOT TESTED, accessible from `youtube_search_requests.extractor.ChannelExtractor` and `youtube_search_requests.extractor.ChannelAsyncExtractor`)
- fixed failed to get related videos in VideoRelatedAsyncExtractor class
- added external cookies support

- fixed async process

- reworked module
- added playlist search
- added related video search
- improved search
- added filter argument in video searching
- added language argument in YoutubeSearch and AsyncYoutubeSearch

- fixed failed to get session id in YoutubeSession and AsyncYoutubeSession
- added shortcut cli (see README.md for more info)

- reworked how async works in youtube-search-requests
- [NEW!] safe search feature, it allow you to filter mature videos.
- added more user-agents in constants.py

- Fixed critical issue when importing youtube-search-requests module (module 'aiohttp' not found)