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

mkdocs/sitefiles.txt

@@ -5,3 +5,4 @@ embed.html
 webmanifest.json
 favicons
 ruffle
+adblock