Add Referer header to Pixiv defaults to reduce 403 responses

[m6man]

For some posts Pixiv responds with a 403 on webserver layer (nginx response).

I assume this is a measure against hotlinking all images of a gallery with ~14+ images on another website, might be a simple regex in nginx to require this header for /img-original/...p15.img and upwards (not further analyzed).

During investigation i came across this extension which also simply adds this header to direct requests to media:
https://chromewebstore.google.com/detail/pixiv-image-extension/cpacgkdedbhileoelnmdegkedcoammhg

This is a draft PR until i've investigated other failed downloads or until i'm sure this change actually helps.

[Bionus]

So that extension adds the referer header with value "http://pixiv.com/" on the pximg.net domain, but your change will add the Pixiv header instead. Shouldn't it be referer?

Also, the "referer_image=page" value in the settings will automatically set the Referer header to whatever URL is the current search page using. You could use the domain with "referer_image=host" instead, but would that really help?

Did you meet any "403 forbidden" errors when using Grabber?