Markdown filter: make it possible to override configuration of HTML subfilter MD filter uses

[Kuro Kurosaka]

Markdown filter uses the HTML filter as its subfilter to parse the HTML contents embedded in Markdown documents. The configuration of the HTML filter is currently fixed to the one contained in the Markdown filter.

There is a use case where the user wanted to make the <pre> section protected from translation, i.e. not extract. This could be done easily by overriding the rule for the pre tag in the HTML filter's configuration.

This ticket is for the feature enhancement to allow the user to specify their own HTML filter configuration file.

[Chase Tingley]

@ssikuro For an example of this, look at how the AbstractMarkupFilter code handles cdataSubfilter and pcdataSubfilter.