Adblock Support for Embeds (#308)

Fixes #307 

- Adds optional adblock support, via wabac.js (dependent on
webrecorder/wabac.js#167).
- For now, adblock enabled only for embeds via the `useAdblock`
attribute, and can be enabled per archived item.
- By default, adblock filter rules from
[EasyList](https://easylist.to/easylist/easylist.txt) are used, however,
a custom filter rules file can be used via `adblockRulesUrl` attribute
(default is ./adblock/adblock.gz)
- Added `fetch-adblock.sh` script to fetch latest easylist, filter out
`##` rules and compress to `./adblock/adblock.gz`
- Not yet possible to enable adblock for standalone browsing via
replayweb.page
- bumps version to 2.0.0-beta.3

.gitignore

@@ -12,6 +12,7 @@ coverage/
 **/__pycache__/
 dist/
 plugins-*/
+adblock/adblock.gz
 !mkdocs/site/docs/examples/*.warc
 mkdocs/_genhtml/
 mkdocs/site/index.html
@@ -19,4 +20,5 @@ mkdocs/site/embed.html
 mkdocs/site/webmanifest.json
 mkdocs/site/favicons
 mkdocs/site/ruffle
+mkdocs/site/adblock
 ipfs-core.min.js