Add a way to retrieve streams without decrypting all of them

[d60]

Hello,

First of all, thank you very much for developing this library.

While reading the source code, I noticed that calling YouTube.streams decrypts all the streaming urls. Since decrypting a url takes some time, this could lead to a loss of time if all decrypted urls are not actually needed.

My suggestion is to introduce a new property (e.g., YouTube.encrypted_streams)and new classes (e.g., EncryptedStreamQuery and EncryptedStream), where the url decryption would only happen when calling EncryptedStream.url.

I apologize if I misunderstood anything. Thank you for your consideration.