Feature/translating transcripts

[jdepoix]

• added list_transcripts(video_id) to YouTubeTranscriptApi, which returns a TranscriptList object, representing all available transcripts
  • TranscriptList objects provide the methods find_transcript(languages), find_generated_transcript(languages) and find_manually_created_transcript(languages), which return Transcript objects
    • Transcript objects represent a transcripts and provide metadata properties
    • by calling transcript.fetch() the actual transcript is fetched
• added new translation feature
  • by calling transcript.translate('de') on a Transcript object YouTubes translation feature can be accessed
• added more specific exception types and thereby vastly improved error messages
  • TranscriptsDisabled, NoTranscriptFound, VideoUnavailable, NotTranslatable, TranslationLanguageNotAvailable, NoTranscriptAvailable
  • all exceptions are subtypes of CouldNotRetrieveTranscript
• updated CLI to make new features accessible
• updated Documentation to explain new features and changes
• completely reworked how the module scrapes transcripts, which makes it way more solid

BREAKING CHANGES:

• youtube_transcript_api.YouTubeTranscriptApi.CouldNotRetrieveTranscript has been move to youtube_transcript_api.CouldNotRetrieveTranscript