New metadata fields for tumblrgallery.xyz and fix id field

[dajotim937]

FIXED 1. By default, tumblrgallery extractor add id field to metadata and default filename template has this id field in it.
But there are 2 (that I notice) situation when id field get wrong value.

Examples

Here is 4 json metadata output, first 2 correct and 2 other are wrong:
Correct:

    "category": "tumblrgallery",
    "extension": "gif",
    "file_url": "https://78.media.tumblr.com/49cd89ee1fa8d8fa8416207e99e93cb4/tumblr_p5erggBa4w1qkiosfo1_1280.gif",
    "filename": "tumblr_p5erggBa4w1qkiosfo1_1280",
    "gallery_id": 124632,
    "id": "p5erggBa4w1qkiosfo1",
    "num": 14,
    "subcategory": "tumblrblog",
    "title": "o0st0ned0o"
}
{
    "category": "tumblrgallery",
    "extension": "jpg",
    "file_url": "https://64.media.tumblr.com/07fc0519adf801d771b29ab1d6c815b6/tumblr_mwo1qzhOa21qze9qao1_r1_1280.jpg",
    "filename": "tumblr_mwo1qzhOa21qze9qao1_r1_1280",
    "gallery_id": 15576,
    "id": "mwo1qzhOa21qze9qao1",
    "num": 20247,
    "subcategory": "tumblrblog",
    "title": "katgrl64"
}

Wrong:

{
    "category": "tumblrgallery",
    "extension": "jpg",
    "file_url": "https://78.media.tumblr.com/618eb08d9ab417cfa0c983ea83f34820/tumblr_inline_p4oaefZvDE1ree08q_1280.jpg",
    "filename": "tumblr_inline_p4oaefZvDE1ree08q_1280",
    "gallery_id": 124632,
    "id": "inline",
    "num": 3,
    "subcategory": "tumblrblog",
    "title": "o0st0ned0o"
}
{
    "category": "tumblrgallery",
    "extension": "gifv",
    "file_url": "https://78.media.tumblr.com/d2c552db301178f9cf93eec685b82ff6/7ae723c87e312dcb-27/s500x750/b81ff566403dca3ba8f341fdf1f2e2b5d9862629.gifv",
    "filename": "b81ff566403dca3ba8f341fdf1f2e2b5d9862629",
    "gallery_id": 124632,
    "id": null,
    "num": 40,
    "subcategory": "tumblrblog",
    "title": "o0st0ned0o"
}

---

2. Also, sometimes content\image in gallery have description and link to post. Would be nice to have in metadata description, post id, content\image position number in post (for now there is num property which hold content position at first url, so for gallery link it's content position in gallery and when download post it's position in post) and previous blog name where picture reblogged from when it's available .
   For example:
   https://tumblrgallery.xyz/tumblrblog/gallery/50312.html
   First one image has this corwinprescott:“Pulp”Seattle, Wa 2017Corwin Prescott - Gothlet... in description and also link to this post https://tumblrgallery.xyz/post/49178.html, where image is first of six in post.
   Or here https://tumblrgallery.xyz/tumblrblog/gallery/50312/7.html ; 2-9 images are in one post with description.
   Both posts refers to another blog, so don't really know which blog name should we put in title metadata field. For now title has blog name from https://tumblrgallery.xyz/tumblrblog/gallery/<gallery_id> (so submissive-monarch in this case). Maybe can add one more field for "original" blog name where picture reblogged from.

3. gallery-dl already support downloading posts. I propose to fix title field, because now it has title of page, and usually (when it has " - " split string and below "see more : ") it consists from blog name and description, so maybe they should be split to title (blog name) and description. If it doesn't have " - " like here, then leave it without split.
   Also technically gallery_id contains post id, not gallery.

4. Primary blog name or url. Like in https://tumblrgallery.xyz/tumblrblog/gallery/50312.html this gallery primary blog was submissive-monarch.tumblr.com , so could add url or just submissive-monarch