[Feature] Padded page_index and page_number formatting options

[NHOrus]

Prerequisites

• Did you read FAQ section in readme.md?
• Did you test with the latest releases or commit ?
• Did you search for existing issues in Issues?

Description

Some image viewing software doesn't support natural sorting order - with parsing numerals and ordering pictures. It makes ordering of manga pages when viewing them - in pathological case - go 0; 1; 10; 100; 11; 12... which is disorienting

Sadly, I'm not always able to use sane image viewer, so pixivutil2-side workaround would be useful for this cases.

Please implement padded_page_index formatting that makes numbers go 000; 001; 002;... 009; 010 and so on.

[Nandaka]

it should pad with 0 and start from 1 if you use %page_number%
https://github.com/Nandaka/PixivUtil2/blob/master/PixivHelper.py#L233

[Nandaka]

example:
filenameMangaFormat = %searchTags%\%member_id% %member_token%\%R-18% %urlFilename% - %page_number% - %title%

result:

[NHOrus]

Oh. Maybe correct readme? There's nothing about padding in there right now