Streaming support

[zcorpan]

See whatwg/html#8759 (comment)

The HTML parser spec is streaming (although the tree is sometimes mutated in interesting ways). In whatwg/html#2142 there's a proposal to add a streaming API for the parser.

If the sanitizer API should be usable with the streaming HTML parser API, then sanitization needs to also be streamable.

It seems possible to perform sanitization in the tree builder stage of the HTML parser: https://html.spec.whatwg.org/multipage/parsing.html#creating-and-inserting-nodes

[annevk]

Since parsing and sanitizing are tightly coupled in the current APIs this is no longer a concern.