Add a settings page for markdown plugin

[Leilei332]

The table in $:/plugins/tiddlywiki/markdown/config isn't convenient to edit configuations for markdown plugin, and it also causes the page to overflow in mobile devices.

This PR add a settings page for markdown plugin under the "Settings" tab in control panel, which is convenient to edit settings. It also removes the overflowed table in $:/plugins/tiddlywiki/markdown/config.

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Confirmed: Leilei332 has already signed the Contributor License Agreement (see contributing.md)

[pmario]

It looks good. But I would move the "Quotes replacement" section below the "Typographer" toggle. Quotes depends on Typo- So they should be together

[pmario]

There should also be a link to eg:

For more information see the plugin "readme and config" tab -- (Using TW wiki-links )

[pmario]

If we have settings. Imo config is redundant

[Leilei332]

If we have settings. Imo config is redundant

IMO the contents in the config tab can be moved to the readme tab.

[Jermolene]

IMO the contents in the config tab can be moved to the readme tab.

I think that's a good idea @Leilei332

[Jermolene]

Hi @Leilei332 I'm going to merge this now, we can always come back to moving the config tab content into the readme tab.