Handling markdown extensions

[fmatter]

I've grown the list of used extensions quite a bit, due to the rather complex document I am getting from pylingdocs:

    md = Markdown(
        extensions=[
            TocExtension(permalink=permalink),
            "markdown.extensions.fenced_code",
            "markdown.extensions.md_in_html",
            "markdown.extensions.tables",
            "markdown.extensions.attr_list",
            "markdown.extensions.footnotes",
        ]
    )

Right now I was going to add this subscript extension, which got me thinking: 1) the markdown function should optionally accept a list of extensions 2) maybe some of the extensions above should not be used by default?

[xrotwang]

I reallize that everything I said at https://github.com/clld/clld-markdown-plugin/pull/7#discussion_r1011287128 should go here. So yes, I agree with

• configurable list of extensions
• principled selection of default extensions

[xrotwang]

see #7