Add Transcript & Channel class

[mytja]

So, YouTube has added transcription (slovenian "Prepis"). Now we have Transcription classes with really minimal code.

Have added tests, but adding you as reviewers as you never know what you can find 😉.

[alexmercerind]

Works for me! 💚

Well, we surely have a lot of features in this library.

[raitonoberu]

It works well, great job! Just a couple of comments:

• If subtitles are disabled in the video, it raises the Failed to extract key exception. It would be nice to add a proper way to handle this. Maybe just return an empty dict?
• Now it returns transcriptions in English. How about adding a language selection option?

[mytja]

• If subtitles are disabled in the video, it raises the Failed to extract key exception. It would be nice to add a proper way to handle this. Maybe just return an empty dict?

This I will certainly fix, but I haven't got an example of video like this.

• Now it returns transcriptions in English. How about adding a language selection option?

Will have a look.

[raitonoberu]

I haven't got an example of video like this.

Mostly various music videos (not sure how it works): https://www.youtube.com/watch?v=hzpt3fQjY9U

[mytja]

Done.
Now user can retrieve different languages using params parameter, which I give them for specific language.

[mytja]

I've also added Channel class, which displays channel info - Resolves #164

[mytja]

Will merge this tomorrow