Improve markdown support

[fynnlyte]

See https://github.com/perfood/couch-auth/blob/master/src/template-utils.ts#L11

• Make the Styling of URLs customizable via the config
• Add support for more Markdown features, e.g.code blocks. But keep things simple, I don't want an external dependency for that.
• Remove the Markdown markup from the plain text version that is sent out
• Add tests