[2ch] add 'thread' and 'board' extractors

- [2ch] add thread extractor
- [2ch] add board extractor
- [2ch] add new entry to supported sites

docs/supportedsites.md

@@ -13,6 +13,12 @@ Consider all listed sites to potentially be NSFW.
 </tr>
 </thead>
 <tbody valign="top">
+<tr>
+    <td>2ch</td>
+    <td>https://2ch.hk/</td>
+    <td>Boards, Threads</td>
+    <td></td>
+</tr>
 <tr>
     <td>2chen</td>
     <td>https://sturdychan.help/</td>