WIP: HTML API: Allow extending input document for chunked processing.

[dmsnell]

This work is still at an early stage. It will change, possibly substantially, before it's ready for full review.

Description

In some situations it may be useful to process an HTML document as it's being generated. This requires the ability to add chunks of text to the end of the input HTML.

In this patch the WP_HTML_Tag_Processor::extend_input( $html ) method is providing this ability. It is designed to allow for splitting an HTML at arbitrary boundaries.

$p = WP_HTML_Processor::createFragment( '<div><p><em>This</em> is <str' );
while ( $p->next_tag() ) {
	// Process everything up to this point.
}

$p->extend_input( 'ong>incomplete.</strong></p><img></div>' );
while ( $p->next_tag() ) {
	// Continue processing with the full contents.
}