HTML rewriting: also rewrite `poster` attribute

https://github.com/openzim/python-scraperlib/pull/338 identified that when rewriting HTML, we do not rewrite of the `poster` attribute.

This should be added because it is clearly just a detail we have overlooked IMHO (and https://github.com/openzim/python-scraperlib/pull/338 changes adapted accordingly to also handle the `poster` attribute).