Update to new Niconico playlist format

[viliml]

I used pages since niconico has native support for them and requests for more than 200 videos at once sometimes fail.
I didn't bother with submitting a PR to the original repo since there's no point in trying to download a playlist if every video fails many times because of the lack of heartbeat.
I am the original author of this code and I am willing to release it under Unlicense.

[tsukumijima]

Thank you for the pull request! (I'm sorry I was late to notice)
I knew that Nico Nico's My Page was renewed, but I didn't know that the format of My List had changed.
I haven't tested this code yet, but it doesn't seem to be a problem, so I'd like to merge it once.
If there is any problem, I would like to fix it in the future.
(Since we are using translation, the sentence may be unnatural)