[pixiv] include ugoira_meta in zip

[TestPolygon]

The program currently downloads only zip with frames, but there is ugoira_meta.json (https://www.pixiv.net/ajax/illust/${id}/ugoira_meta) that contains meta data about the animation — the delays that are required for the correct FPS.

Maybe it makes sense to include this json in zip?
For example, Pixiv Toolkit includes body.frames array from this json as animation.json file.

[TestPolygon]

By the way, there is frames[][delay] property for ugoira files, but this does not work for the purpose of filenaming: [pixiv][error] FilenameFormatError: Applying filename format string failed (ValueError: Empty attribute in format string).

[TestPolygon]

I have tried to save the delay frames[0][delay] in HTML generated by postprocessor, but ?:// operator does not work with it.
{frames[0][delay]?:<br>Ugoira delay: //}

It shows [postprocessor][error] 'metadata' initialization failed: ValueError: Only '.' or '[' may follow ']' in format field specifier.

[mikf]

You mixed ? and :. Try {frames[0][delay]:?<br>Ugoira delay: //}.

It is also now possible to set a filter for post processors (3cbbefd), which, according to #1460 (comment), might help with this issue here.

[TestPolygon]

{frames[0][delay]:?<br>Ugoira delay: / ms/} works.

But is it possible to escape /?
{frames[0][delay]:?<br>Ugoira delay: <span class='delay'>/<\/span> ms/} does not work.

[pixiv][error] An unexpected error occurred: ValueError - Invalid format specifier.