Introduced htmlParser.fragment#filter and htmlParser.filter#applyTo.

Conflicts:
	core/htmlparser/element.js
Reuse fragment#filter in fragment#writeHtml.

Conflicts:
	core/htmlparser/fragment.js
Filter children when filter passed for fragment#writeChildrenHtml.
Implemented comment#filter.
Restored ability to filter children before parent - filter is applied only once to each element.
Improved documentation for htmlParser.*.
Restored 2ng argument for writeHtml methods.
Merge branch 't/9981' into major