Add support for HTML comments

[skilar]

This is a great package! Thanks for writing it.

I've encountered some HTML with comments, such as <!-- Main -->, and always get the following error: undefined method 'extract_span_xml_comment'. It would be helpful if kramdown-prismic could support this in some way instead of failing. I don't actually need the comments converted — stripping them would be all right.

[francois2metz]

Thanks for reporting it @skilar. This is partially a regression of the 0.3 version. I have released the version 0.3.3 to fix it.

[skilar]

That was fast! Wow! Thank you, @francois2metz!