Improve HTML filtering rules.

[ngorskikh]

We've added some new functionality to HTML filtering rules in CoreLibs, see discussion and updated HTML filtering rules description.

The same improvements should be implemented in tsurlfilter. However, @ameshkov asked to mention that tsurlfilter should probably support the full CSS selectors syntax/semantics and all of the extended-css pseudo-classes instead, since it should be straightforward to implement.