Pass args to fast html

[Kuret]

For testing partials with fast_html, we needed to pass arguments to its parse method. Since Floki did not support passing arguments to their HTML Parsers an option to pass arguments to the parser module was added.

Note that this only has effect on fast_html, since both mochiweb and html5ever do not support additional arguments.

This was needed because (for example) we had a partial for a table row. When testing this partial on it's own, the output would ignore the tr and td element, since it was 'invalid html' as the parent table element was missing.

This can be solved by passing a context: "table" to fast_html's parse function, only Floki did not support passing along arguments.