[Enhancement] Some sites download image by sending POST

[chinggg]

Some sites, such as https://stocksnap.io download image by sending POST request to a fixed url (in this case https://stocksnap.io/photo/download/) with photoId and _csrf. The csrf token is hidden in the html of each individual site and can be extracted from <input type="hidden" name="_csrf" value="">.

How to implement SearchExtractor under the circumstance? Since job.run() will iterate over extractor.items() and dispatch each msg to handle_url(), which then called download(). But It seems the HttpDownloader is always sending GET request. (see https://github.com/mikf/gallery-dl/blob/master/gallery_dl/downloader/http.py#L122-L125)

[mikf]

This is now possible by setting _http_method and _http_data in a file's metadata dict.
(be34927, c0c1277)

For https://stocksnap.io/, it would be

file["_http_method"] = "POST"
file["_http_data"] = {
    "photoId": "…",
    "_csrf"  : "…",
}