add extractors for fantia and fanbox (#1459)

* add extractors for fantia and fanbox

* appease linter

* make docstrings unique

* [fantia] refactor post extraction

* [fantia] capitalize

* [fantia] improve regex pattern

* code style

* capitalize

* [fanbox] use BASE_PATTERN for url regexes

* [fanbox] refactor metadata and post extraction

* [fanbox] improve url base pattern

* [fanbox] accept creator page links ending with /posts

* [fanbox] more tests

* [fantia] improved pagination

* [fanbox] misc. code logic improvements

* [fantia] finish restructuring pagination code

* [fanbox] avoid making a request for each individual post when processing a creator page

* [fanbox] support embedded videos

* [fanbox] fix errors

* [fanbox] document extractor.fanbox.videos

* [fanbox] handle "article" and "entry" post types, all embeds

* [fanbox] fix downloading of embedded fanbox posts

docs/supportedsites.md

@@ -151,6 +151,18 @@ Consider all sites to be NSFW unless otherwise known.
     <td>Chapters, Manga</td>
     <td></td>
 </tr>
+<tr>
+    <td>Fanbox</td>
+    <td>https://www.fanbox.cc/</td>
+    <td>Creators, Posts</td>
+    <td><a href="https://github.com/mikf/gallery-dl#cookies">Cookies</a></td>
+</tr>
+<tr>
+    <td>Fantia</td>
+    <td>https://fantia.jp/</td>
+    <td>Creators, Posts</td>
+    <td><a href="https://github.com/mikf/gallery-dl#cookies">Cookies</a></td>
+</tr>
 <tr>
     <td>Flickr</td>
     <td>https://www.flickr.com/</td>