Add support for piapro

[quozbogv]

piapro is a Japanese website made for hosting VOCALOID-based media, mainly images, audio, and text.

Example URLs for audio:
http://piapro.jp/content/es7uj48x6bvcbtgy (old-style URL, still functional)
http://piapro.jp/t/KToM (current URL style)

Two types of audio quality, download and streaming. Very similar to #5839, for the download needs one to be logged in, while the streaming doesn't.

Download: http://piapro.jp/download/?id=es7uj48x6bvcbtgy&view=content
Streaming: http://c1.piapro.jp/amp3/es7uj48x6bvcbtgy_20100116020522_audition.mp3

The "es7uj48x6bvcbtgy" and "20100116020522" parts can both be derived solely from the HTML source:

<input type="hidden" name="id" value="es7uj48x6bvcbtgy">
                                      ~~~~~~~~~~~~~~~~

<a class="link_songle" href="http://songle.jp/songs/piapro.jp%2Ft%2FKToM%2F20100116020522" target="_blank">http://songle.jp/songs/piapro.jp%2Ft%2FKToM%2F20100116020522</a>
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It might be ethical for youtube-dl to have the user know of the license agreement for a song, which is contained in <ul class="plc">.

[szc126]

I believe I have written a working and sufficiently comprehensive solution (piapro.py).

• There are still XXXs and TODOs.
• It also has the ability to download images, texts, and 3D models that are also hosted on piapro. This is probably very unorthodox, but adding the extra support was nearly effortless, and PiaproIE thus covers all 4 upload types. I hope that it is alright.
• I have also written a PiaproUserIE.

I humbly await a review and suggestions.